In Ethereum there's one disincentive to stake pooling: if a staker misbehaves, perhaps because it got hacked, the penalty depends on how much stake misbehaves at once. A large amount of misbehaving stake could be completely destroyed, while a small amount will lose only a small portion.

This is mainly because a small amount does no damage and is presumed to be accidental, but also helps decentralize since you're safest not using a large staking pool, the most popular client, the most popular hosting service, etc.

Running as a small staker is feasible because the minimum stake is only 32 ETH, you're profitable if you're online at least 2/3 of the time, and the load on an individual node is feasible for a laptop. The computational load increases fairly linearly with the amount of stake, so there's little economy of scale for a large staker.