The claim that I've read and heard from linguists about this is that while words like janpusuru are semantically verbs, grammatically they are a noun janpu + the standard verb suru.

Contrast the English expression "I am doing homework" vs. "I am *homeworking". "Homework" isn't really used as a verb in English, but we can express the idea of homework-as-an-action by saying "do homework".

New non-suru verbs in Japanese do apparently happen from time to time (Wikipedia uses the example of guguru — "to google") but they're rare, so the class is mostly closed.