So why should a phone be different
Off the top of my head, because people tend to download a lot more apps onto their smartphones than they do their PCs (where many tasks are carried out over the browser instead), and phones tend to be host to way more personal information, so one can argue that it ought to be subject to tighter controls.
The second reason is that the open nature of the internet has not been without its share of problems (eg: viruses and malware), so I am not actually against the idea of things being done differently.
The question shouldn't be "PCs have always been this way, so nothing should change and the iPhone should follow". The question ought to be "Yes, PCs have been this way for decades, but what's so good about the old ways that justify things not changing and making the iPhone to be more like it, rather than the other way around?"
For instance, when you have a few hundred apps on your phone, I imagine it is still easier to manage all of them within a centralised App Store where you can update them all at one shot. If I have a new phone, it's a simple matter to go into the App Store, check out my list of purchased apps and tap on the ones I wish to download and reinstall.
Can you imagine buying a new phone, then having to visit a hundred different websites to download the apps you need again? Imagine if they requested for a receipt or key that you no longer had available (which was precisely what had happened when I upgraded my Macbook and wanted to download Airbuddy again and was like, whoops, where's my registry key?).
There is also the added security to be had from using iTunes billing over keying in your credit card information into multiple different websites (I wonder how Amazon is able to get away with not needing an OTP on their website while other online shopping websites like Shopee do). I just wish more websites and businesses would support Apple Pay in the very least. It would save me the hassle of needing to create a new account with them just for a single transaction.
It would also be nice to be able to manage all my subscriptions from within one app. Recently, I had to cancel my old credit card and apply for a new one, which amongst other things, meant having to visit a dozen different websites in order to update my payment details (luckily, I track them all within an app). For the ones which I am subscribed in-app, it's simply a matter of updating Apple Pay and letting Apple handle the rest. Cancelling my subscriptions would also be as simple as tapping a button in the App Store app. No need to visit their website ever (and some don't really make easy for you).
I think Apple has done a lot of work to make managing all these drudgery as seamless and convenient as possible, but I guess a lot of it wouldn't work with the decentralised nature of the internet. And this is possible only because we as consumers give Apple a lot of influence over developers and force them to capitulate to demands which are often against their own self-interests (even if they are to our advantage).
I guess this is why I am actually not against the idea of a walled garden. Because in this new world order, we have the power, not developers.