Although I agree that FAT and FAT32 definitely have their disadvantages, I wouldn't say they are the worst filesystems ever made. (Btw, DOS 1.0 only had the root directory and no subdirectories, I think that was way worse.) Both FAT and FAT32 are very simple and can be implemented quickly and have only very limited requirements on CPU and RAM resources. Nowadays, there still exist devices for which this is an important advantage ("internet of things"), although it's probably getting less important, considering that something as small and cheap as a Raspberry PI runs a complete Linux.
I don't know the answer why Windows and MacOS do not support ext4, but I guess it is because Microsoft and Apple are not interested in supporting it. A feature always comes with costs: It has to be implemented, tested, updated to new versions of OS/drivers/other software, tested again etc. I think that most users of Windows and MacOS do not care accessing ext4 partitions, and many of them probably even don't know what ext4 is.
And there always is a trade-off: The people that are busy implementing/testing/updating feature A cannot work at the same time on feature B. But if feature B is more "important" than feature A, you would rather have feature B implemented instead of feature A.
Now imagine you would work for Apple or Microsoft: Would supporting ext4 make sense, would it be worth the effort? Probably not. Would you rather have your programmers implement something else, maybe something that's going to be the next cool thing or will help earn a lot of money? Probably yes.
Actually, there are ways to read or write ext4 from MacOS or Windows: ext4 on Wikipedia, How to read ext4 partitions on Windows on superuser.