TV-Nihon

Kamen Rider has had

Spoiler

three series in a row now where the Final Boss is, at least technically, an evil Kamen Rider.

If the writer is too lazy to create an original villain, then just make the bad guy a darker version of the main character.

Maybe it's easier for them to create an evil Kamen Rider rather than design something completely different. From a toys standpoint, an evil Kamen Rider is a nice collectible rather than let's say some random monster of the week, then again there have been exceptions where the big bad boss gets a figure too and is also good for collection. Maybe it's on them, maybe complacency is setting in..

An evil rider as the final boss means they can release a driver toy around the Q3 time, which is usually the last little bit they release toys for the season and probably encourages people to but them.

Cronut and Evol where both super popular, so they are onto something.

You can also sort of blame Black RX for that. Since Shadowmoon was basically an evil rider. It's a famous arc. I think Sagittarius was Rider-Esque form Fourze too.