The simple reason is because it's not my fight. Acceptance of evolution is the mainstream Christian view. At least for Christianity, a literal view of Genesis is only held by some weird cults, mainly just in the USA. If a Christian wants to debate evolution they can discuss it among themselves. There are more Christians who accept evolution than there are atheists.
But more than that, the creationists have an ulterior motive trying to bait atheists into debating evolution. They don't care if they win or lose so long as they can keep framing this as an atheist vs religious debate to their fans. Don't feed the trolls. It doesn't mean we have to allow misinformation to go unchallenged, it's just that we lose by winning anyway. Direct them to other Christians. This isn't really a science debate, it's a doctrinal debate within Christianity. They can work this out themselves.