This is by far the hard part. The complete rulesets for most games[0] would be smaller than a prompt necessary to turn that board game into a computer game[1].
Anyone can draw some counters and a board, make up some rules. Making those rules interesting, let alone fun, is so much of a problem that many published board games get house-ruled in various ways[2].
The AI-made games I've noticed (clichés, the AI can't help it) have been at the median level of the old Flash sites, but with less variation: fewer total disasters, but also nothing amazing.
While this isn't great, it's also not nothing: I wouldn't be surprised if one of the current ones goes through an iterative refinement like Bloons TD did.
[0] not all: I was obsessed with Star Fleet Battles as a young teenager, these days I describe it as "A computer game written about ten years before home computers had enough memory to store all the rules", which is why each turn takes 2 hours, one of which is counting out boxes and using that to fill out a spreadsheet as you work out how much power your ship has and how fast it's going and which equipment you can use, and distributing weapon damage on some target is worked out with the help of an A4-sized printout that functionally implements a dice-driven state machine.
[1] Unless the prompt was "here is a photo of the rules, turn it into a video game", but I count that as cheating, "a picture is worth a thousand words" etc.
[2] My main house rule for Star Fleet Battles, these days[3], is to simplify the 4+ kinds of power source into one. I can appreciate the world building of "an impulse engine can't divert antimatter to a photon torpedo", but it's not actually fun to keep track of this.
[3] I've played it like 4 times in the last 15 years.