White paper of Bitcoin was written to introduce this p2p payment system and that's why the halving was not mentioned in it. OP, if you're part of Bitcoin community then you might know that Bitcoin has gone through some changes overtime like introduction of Segwit addresses in 2017 to the source code. Satoshi Nakamoto hasn't introduced that address but it was introduced by other developers and Bitcoin community supported it.

Similarly the halving event was not mentioned in Bitcoin whitepaper but it was already part of Bitcoin source coin from the start and overtime people learnt about it and community supported it. It's the halving event that has allowed Bitcoin to gain value overtime and I don't think anything is wrong in not mentioning it in the white paper. I think it wasn't even needed to mention it in white paper and that's why Satoshi avoided mentioning it.