Because modern hardware includes instructions that old software could not benefit from. For example H.264 video decoding has specific instructions physically designed into the silicone to speed up playback that isn't there on older hardware.
That being said, it really depends on the use case and a lot of "modern" software is incredibly bloated and could benefit from not being designed as if RAM is an unlimited resource.

It's an old game but I recently discovered Kittens Game. I also love Balatro, but Kittens it is more complicated and deeper. It's basically balancing spreadsheets for fun. Don't look at the wiki until you can get 120 kittens.