I suspect there isn’t money in it. You can’t charge a subscription for a keyboard, so what else is there to do. Well you can and I think Grammarly has one with a subscription, but that’s a niche market.Is there something going on with third party keyboards? Google barely updates GBoard for iOS anymore and its gotten very buggy. Are they just too much of a hassle to maintain?
SwiftKey had themes you can buy, but I don’t suspect they sold very well. It’s probably extremely difficult or impossible to use it for a keylogger, so there goes any kind of data selling revenue. You can’t add actual ads in the keyboard, so no money there. There’s probably just more profitable apps to create for sale in the App Store.