[Annuga]

Other reasons have already been explained but here's a few reasons I've got to add
"why did RPM need cars to fight Venjix?"

Your argument here is pretty much "You should watch better shows instead! It's your fault for expecting too much" when not only was Megaforce supposed to be an anniversary series but had the beloved Gokaiger as source material.

"how did the Ninja Nexus Star fall from space when ninjas originate from Japan?"

The reason for the cars is because the source material forced them to do it. Plus this gets lampshaded by Ziggy, the green ranger of RPM and he asked Dr. K, the creator of the stuff in that season about it, I forgot her explanation but they DID talk about it in the show. Can't speak for others as the only other seasons I've seen fully were Ninja Storm and Time Force.
I assume this is Ninja Steel but there's a simple explanation. Soace ninjas. I know there's a Ninninger galaxy in the show's canon so that's that.

"And everyone still says the explaining was bad, just because of the "blitz" legendarys, and the pirates"

The blitz legendaries are a valid complaint as it's supposed to be a Power Rangers anniversary, suddenly having ranger suits the franchise never had before and brushing most of them off isn't a good thing when other seasons get stuff wrong, such as the Samurai disks being used incorrectly or the Wolf Ranger being called the Jungle Fury purple ranger. It's a combination of things.
The pirate one is also valid but for multiple reasons. Gokaiger was beloved not just for being an anniversary season done well, but also because of the likeable cast. Thus, the fact that the pirate themed Sentai are merely a power up isn't just disappointing for Gokaiger fans, it's not just wasted potential as there's so much you could do with a pirate theme, but there's also the fact that a 4 year old can see they're pirates but unless you know of Gokaiger, you won't understand WHY they're pirates. The pirate suits only gave them the ability to transform into other Rangers whereas in Gokaiger, the pirate them was prominent for the story and characters