Because having games designed to run on legacy platforms is simply amazing. Check out what some devs have been developing for the Mega Drive, it's absolutely mind blowing to see people pushing the hardware limits almost 40 years later.

Some people find it enjoyable to try to develop games with limitations, that’s why the pico8 and tic exist

I just got a Dreamcast as a gift and I am super excited about it.
There was also a successful Kickstarter for a modern-day tech drop-in replacement for the VMU (the memory card with a screen and buttons that some games would load minigames onto, similar to tamagotchis and Pokéwalkers).
People are still regularly releasing games for the Game Boy and the Commodore 64 and I welcome new games for the lot of them.
Lots of cool hardware tech that flopped or got dropped, and they're fun and interesting to revisit.

Cool sidenote: the Dreamcast controllers had two slots. I wonder if they had cool interactions, like the MSX, the SuFami Turbo, and the relatively more modern but more distantly related Metal Gear Solid where Psycho Mantis comments on what other games you've been playing based on the saves on your memory card.

Agreed with Magician and RmDebArc, and besides that, it could also be for the challenge, for the enjoyment, or because the vision of the dev only works in such a platform. And with emulators that allow commercial use around, the developer can also publish his games to modern platforms without having to rebuild the whole game.