Why normal bitcoin users can't use their money to run their own full nodes themselves instead?
Bitcoin don't need more full nodes, by the way. 20 pools can handle all the problems with propagation.
It depends on who is a "normal" Bitcoin user for you. It took me about 3 days to synchronize with the network, I guess that "normal" Bitcoin user wouldn't be so patient to wait 3 days "only for better security". Usually they depend on web or light wallets. There are about 11 thousand Bitcoin nodes right now and it looks like it won't change anytime soon due to Lightning Network which also gives you ability to create a node which in fact will earn (small amounts of) money.
I just use OP term. "Normal" look too broad term for me too. By downloading full blockchain users can't really improve security compared to light wallets and running full node has no impact on mining pools or blockchain state at all. 20 bitcoin mining pools in full control of all transactions and other full nodes either accept their blockchain or not. All blocks still will be downloaded from 20 top pools, it doesn't matter how many non mining full nodes propagate blocks.