Because Fourze was about FRIENDSHIP! The problem is you can't have evil rider if you doing a friendship show! It would ruined the concept of Friendship!

Right, because the legitimately evil rider wouldn't accept the Rider's friendship, and lord knows how many people Gentaro tried to befriend who he also knocked around with his fists.

It would make the theme moot.
Sometimes people wallow in self pity and need to get smacked the fuck up to see how things get better.

What do you mean?

A reoccurring theme in Fourze was that people would choose to let their own self doubt consume them.
Character is feeling down over some kind of rejection
Character is given switch and gains power
Character uses newfound power to exert themselves over others
Fourze offers friendship, but turns him down because now they're the ones who's strong.
Fourze knocks them senseless
They accept Fourze's offer of friendship

It's not really told the best way (Too often the character is basically unnoticed by Gentaro until after they become a threat, which makes things feel a little unbalanced), but it's kind of the base for a good chuck of switch users.

I got a fun question for you guys! :)

What If you were chosen by Toei to join the cast of Kamen Rider show and they give you a choice of who do you want to play

A. Main Rider
B. Secondary Rider
C. Supporting Characters
D. Villains

Or

E. Movie Rider

I would say Movie Rider.

I wouldn't be able to resist being a main rider. Especially since I could come back for all the crossover movies!

I'd go with Secondary Rider. I don't have what it takes to be Primary. But hell, I'll give my all as a Secondary.

I'd go for secondary or primary.

If I was secondary, I'd be the anti-hero who sees some primary riders being annoying and not taking things seriously.