It's a lot simpler than that.
Glasses don't look that cool.
That's not to say glasses can't look cool, but when you have a character wearing cool glasses, why not go the full mile and have them be sunglasses for extra coolness?

It's as basic as that :lol

You got me there. Sunglasses are pretty cool.

Thinking about it, Another Agito wore his sunglasses so much they might as well have been normal glasses. :lol

That's cheating and you know it.

I think modern comic book writers came up with the excuse that Clark Kent can hypnotize the public using his glasses in order to conceal his superhero identity. Btw Blade wore sunglasses when he kick Tsukasa/Decade's ass. He act more like Tendou than Kenzaki in his small cameo appearance.

There are a thousand reasons by now.

I think everyone just picks their favorite and runs with it.

Remember, most modern Riders are played by models who were hired for their looks as much, if not more so, than for their acting ability. If any of them need corrective lenses they probably go for contacts to avoid covering any part of their pretty, pretty faces.

I hadn't thought about that but it makes sense, though I can't help but wonder just how many housewives are desperately waiting for a Megane Kamen Rider...

I know none of them would be wearing corrective lenses, but glasses as a whole can be considered part of a fashion trend or "Look."
KR Drake, for example, is a make up artist who is supposed to look chic, so glasses could've been part of his wardrobe, and they could've just been clear plastic lenses (Or empty lenses if worried about a glare, and just had him wearing a frame or something)

Gamel from Os wore glasses in Kabuto when he played a rival makeup artist to Drake.
Speaking of Drake's actor, he was also V3 in the KR Next movie too. Both of his roles are dragonfly riders. KR Ichigo from the KR First movie even made a cameo appearance in Kabuto since Kabuto was the 35th anniversary of the franchise. Too bad both movies bomb and we're back to using the old suits for the Taisen movies. KR Amazons sort of carry that darker tones again more successfully and were included in the Taisen. The only dark rider series in the main continuity are the Black, three movies from the 90's, and Kuuga/Agito.