[Ape]

I think there are a number of reasons why this is. But the first and foremost is toy sales.

Bandai have decided that, for better or worse, the best way to make money from toy sales is to produce massive toylines with a collectable gimmick. This gimmick is stretched to include as many as possible with the intention that people will keep buying the toys in order to "collect them all", a mentality which has served Pokemon well over the years. It's seems they've taken the attitude of "the more stuff there is to sell, the more money there is to make".

They also seem to have a large say in the production of the show itself, as for them the show is effectively a commercial for their toys. Therefore they only want to feature stuff that can be sold to the public. This makes sense from a business perspective, only investing in stuff that you can profit off of is how many of the most successful businesses have operated. To add to this profit attempt the show has a limited budget (I suspect they may have learned off Saban that keeping the show budget low is a good way to profit more off it overall).

Week to week monster designs, while cool, just aren't going to be viable long term toywise. Recurring antagonists however are because you can then recreate the fights from the show with the toys. Hence having characters like Machine Chaser, Kamen Rider Genm, Kamen Rider Chronos Night Rogue and Blood Stalk makes sense as they can serve as the character that gets fought and also be sold as a toy.

I do agree though, it does get a bit ridiculous how many times these characters are "defeated" and yet it's never considered over until a specific point in the show when everything feels like it's more intense.