Not quite old ROMs, but gog.com sells old computer games prepackaged for Dosbox, which because of that work on Windows, Mac and Linux. That's basically the old PC computer equivalent of what I believe Nintendo does by shipping the emulator with the ROM when you buy it through the Virtual Console so it runs as a whole.
Back in the day I was a big fan of an SSI game called Imperialism - this game pretty much refuses to run on modern software - it needs DOSBox to run smoothly and even then it does custom cursor stuff that tends to screw up very obviously on modern systems - the GOG version of the game runs smooth like butter.
Why would I ever pirate a copy of Imperialism and spend a day actually getting it set up to run sorta decently on my machine - when I can grab it off GOG for 1.89 CAD? A day of my time, even an hour of my time (even my leisure time), runs well above 2$ at this point - the convenience is there so pirating becomes a bad value proposition.
They sell old games, e.g. Sonic the Hedgehog: https://store.steampowered.com/app/71113/Sonic_The_Hedgehog/
That's a Windows program that runs a Sega Genesis emulator that loads the game's ROM.
In many jurisdictions (AU, UK), it would be legal to copy them to another device you own, such as a hacked PSP, under the "format shifting" exemptions.
Licensing hell.
https://kotaku.com/the-sad-story-behind-a-dead-pc-game-that-...
https://www.rockpapershotgun.com/no-one-will-sell-no-one-liv...
Great games. Recently - well a year ago - played through them with a friend coop.
But I agree with you, I'm surprised Steam doesn't have a way to get old ROMs.