Because, as you see even in THIS thread on a Apple fan site, populated by plenty of "Apple is always right" people, it bugs some of these people that Apple has removed essential use accessories. Not all buyers have a drawer full of these from past purchases. Think of all of the first-time iPhone/iPad buyers for example.
No need to aggravate such people for such low-cost extras. Customer goodwill is worth a little cost. If Apple believes that customers 100% support this move for environmental reasoning, then no Apple fan will request a cable or brick... certainly not the "Apple is always right" segment since they can clearly see that Apple doesn't want to provide them. If all buyers support the "why?", nothing would structurally change in the transaction... EXCEPT Apple wouldn't look so bad to those who take offense for this choice being made FOR them... basically requiring such customers to then spend more money if they don't happen to already have a suitable brick or cable on hand.
Those who may actually need one or both could get them and thus see Apple more favorably vs. seeing Apple as making what can be easily viewed as a pure penny-pinching, greed play spun as an environmental decision. All greed-based plays made by any company get spun as a positive- that's the game of spin, sometimes referred to as "putting lipstick on a pig." A simple change in policy makes all buyers happy about THIS issue... and reduces GOVernments feeling the need to step in and force a very rich company to deliver the mix of stuff to make basic usability work for consumers.
Personally, I have more than a few bricks and suitable cables myself. So I would decline both. But obviously, others need one and/or the other or they wouldn't be so frustrated about it... and/or their GOV wouldn't feel a need to take action against Apple for this kind of thing. So sell customers like me the new <whatever> and we opt out of those accessories like we opt out of all but one color and storage selection. For others who need a cable and/or a brick, give them one if they want one so that they are completely thrilled with their purchase too. It's not like it will break Apple to offer a few optional extras in the name of overall customer goodwill associated with what are typically quite expensive purchases.
As to your suggestion that Apple's version of those accessories are poor quality compared to those offered by others, that sounds like an opportunity for Apple to deliver something better vs. being any logical rationale for leaving them out so that customers who need them can buy a better quality one from someone else. Any such customers can opt to go that way anyway within this concept... or opt to take the "mediocre" ones from Apple WITH the multi-hundred to thousand+ dollar product they are purchasing from Apple right now. It actually seems MUCH better than purchasing "mediocre" anything(s).
Personal sidebar: About 8+ years ago, I walked into an Apple store expecting to lay out the retail price for a MBpro charger replacement because mine had started popping, cracking and sometimes seemed to throw a spark when used in the dark. I think I recall the retail was about $80 or more. And it was outside of warranty.
They asked about the purchase and I mentioned what was happening. They plugged mine in and it makes the odd pop/crack sound. They went in the back and came out with a new replacement and didn't charge anything for it. Impressed the H*LL out of me when they didn't have to do anything. That has stuck with me as a standout example of Apple service for 8 (EIGHT) years as a dazzling show of service after the sale. I've actually told other people about it as part of sharing my enthusiasm for buying Apple stuff.
As I wrote, goodwill goes a LONG way. And so does the reverse of it. I wonder what kind of things people who have spent $500-$1500 for a new thing only to realize they MUST either already have OR SPEND MORE for some fundamental accessories share with their friends about that experience. I imagine the enthusiasm is not quite the same. For what? To save the incredible volume discount cost Apple pockets on a brick and/or cable for that customer in need... accessories Apple makes anyway. To me, the cost of the goodwill harm would not be worth that tiny bit of extra profit in those customer experiences.
A simple, customer-centric, policy change would continue to fully support the environmental push by shifting it to the decision making of Apple product buyers at the point of purchase. Many could opt NOT to take either because they have plenty (or non-"mediocre" ones already). Those who need/want one or both would get what they need/want too. GOVs would have no reason to take actions on this topic because Apple could show they give them to any buyer who wants one. To me, this seems much better than badwill, bad PR and paying lawyers tons of Apple cost to try to defeat GOVs with endless resources.
Lastly, as to these other tech companies potentially making any such move themselves, I'd encourage the same of them. However, as we all argue so very passionately, Apple is BETTER than them anyway. So matching their bad choices because these lessor competitors make any such decisions seems like the WRONG choice. Instead, it offers a terrific opportunity for the "greater than all" Apple to do yet another thing BETTER than those other players. It certainly doesn't hurt for the "most profitable company" demonstrating how RIGHT they do EVERYTHING vs. these inferior competitors we are always looking to put down anyway. Here's one more opportunity to dazzle customers.