Hobbyist hackers have been hard at work on this problem. They have developed hacks that can work around most of the issues in getting more modern MacOS to run on older hardware.
But for older Macs with only older Intel integrated graphics like many older 13-in MacBook Pro models, they GAVE UP. Hackers were able to get newer Mac OS to operate, that was not the issue.
They said that the older Graphics chips in these older Macs were so slow as to be completely unusable for anything, including web surfing and picking up your email.
Because of their antiquated (by today's standards) graphics, these older computers are too slow. Expectations of the basic functions the computer can perform, and how quickly it can perform them, have forced these older computers to the sidelines as far as updating to later versions of MacOS.
But these computers are not dead -- they can still perform the functions they could when you first took them out of the box, and in most cases, have been able to support many versions of software upgrades, and add additional functions.
Apple has decided that these computers are not up to the task of running current macOS, and have simply left them behind for NEW updates. Hackers agree -- they are simply not capable enough. They are NOT abandoned, Apple has just stopped pressing additional capabilities into your hand for free.
If you want the latest features, that will requires a new(er) computer.