Also because Apple designed the chips.
True though the same is true on the phone side and it hasn't stopped them there.
Apple can’t support something designed by another vendor who has dropped support and intel has their own cycle for dropping support for their processors etc.
Yes and no. I am sure Apple is (was) a big enough buyer that they could have gotten Intel to continue support for the select models of the processors they used assuming there wasn't some fatal flaw with the chip. Which 5 years in to these chips it's hard to imagine there is a flaw fatal enough to warrant dropping support in new OS but continuing security support for 2 more years after that.
Then on the flip side, Apple appears to have dropped support for systems that have processors still supported by Intel such as the MacBook Air 2018 and 2019 (i5-8210Y) processor.
Ditto for Motorola previously.
There’s nothing to stop Apple supporting m1 as long as THEY see fit now.
Agree there's nothing stopping them from supporting the M1 for longer but I wouldn't count on it. I don't think they were hellbent on continuing support for older systems all these years but for Intel dropping support for the microprocessor Apple selected.
That said, I now expect Apple to drop all Intel support in macOS 16, which I am assuming is coming ~ September 2025. While I don't think Apple had to do this, it looks like Intel is dropping the processors used in many still supported Mac systems after June 30, 2025:

Changes in Customer Support and Servicing Updates for Select Intel®...
Changes in Customer Support and Servicing Updates for Select Intel® Processors

There are still a few processors used in Mac systems not on that list (e.g. W-2140B as used in iMac Pro but that may be a special case not covered and similarly i3-1000NG4 as used in the MacBook Air 2020), but it's hard to imagine them dropping the Mac Pro (2019) while still keeping all the Intel support to keep the iMac Pro 2017, MacBook Air 2020, and/or MacBook Pro 2020 going.
Then I would be surprised if Apple drops the M1 machines at the same time but I wouldn't be surprised if Apple drops M1 machines from macOS 17. Especially the 8GB machines. Agree with you they wouldn't have to but I wouldn't be surprised...