Buddhism isn’t just about the known world. It contains within it some elements which cannot be verified. There’s no guarantee that it will be possible to verify some of these teachings such as rebirth, kamma etc. as some arahanth attain enlightenment without the ability to see rebirth, devas etc (see Susima sutta) SN 12.70: Susīma (English) - Nidāna Saṃyutta - SuttaCentral.
I say that the Buddha doesn’t go to the extreme of creating significant teachings (such as rebirth or devas) out of thin air as it were. He didn’t incorporate myth into the core teachings- the core teachings were verifiable for everyone regardless of whether they were able to sense the ‘mystical’ teachings suggesting that it wasn’t necessary believe/not believe/be agnostic about those more ‘esoteric’ phenomena. It does also seem that the Buddha while seeing more than others (he said he saw a forest but taught a handful of leave- what was necessary for attaining Nibbana) taught these esoteric bits and didn’t seek to leave them out. I believe this is because they had some usefulness in the practice of the Dhamma. If for some people jhanas and supernormal powers were developed then they would be able to see these things for themselves. It seems that when arahanths were able to do this their views were broadly in line with what everyone sees. However I am not talking about the literary inventions of the later texts here.
The Buddha said he thoroughly verified for himself whether there were devas before he even acknowledge they existed. He doesn’t talk about rebirth or kamma before he came to know kamma and rebirth on his own as far as I recall the EBT. It was a personal search so it wasn’t about teaching karma and rebirth to attract more followers. In fact there is a long debate about rebirth in the DN with a member of the laity who simply couldn’t accept rebirth on the say so of monk. I cannot agree that he simply adopted prevalent cultural beliefs, as he was a seeker of the Truth- an empirical ‘scientist’, and completely disregarded Brahma and Atma (Self) the core teachings that were widespread at the time.