To create or instal a cap, there should have been an inherent way for the Bitcoin protocol and rules to check from who each block comes.

We only know who mines each block because pools and miners choose to actively scream it out, but they could easily not do so. FYI, they do so by editing the op return in the coinbase transaction.

So even if it was feasible to do so, say you manage to create a way where they somehow have to sign it in order to get the coinbase, they could always just use 3-4 different names and then each "name"/entity would have 15%, yet the actual person controlling it would have 60%. They can still hide.

Either way, I think the most pressing issue nowadays is Pooled Centralization, that's why I'm working with the DEMAND Pool team to create and deploy the first Stratum V2 pool. Eventually all pools and miners will use SV2, and then the issue of pool centralization and mining centralization will be largely resolved.