I also think it’s really hard to engineer a good slide phone. Modern smartphones are already really compact. So you either (1) make an affordable slide phone with terrible specs and ok engineering or (2) make a slide phone with excellent specs and engineering but costs a huge amount of money. And I am going to guess most small companies cannot engineer anything like (2) so you just end up with slide phones with bad specs and it’s only selling point is that it has a sliding keyboard. This phone will not sell well.
I believe there is a a keyboard case called Clicks however it appears to only be aimed at iPhones. If it’s a huge deal to you this is one possible solution.