What is the incentive for a bitcoin node to broadcast the newly received block as fast as possible?

[Bitcoin]

What is the incentive for a bitcoin node to broadcast the newly received block as fast as possible?

I understand that a bitcoin node will broadcast a new mined block to all the other nodes as quickly as possible so that the other miner nodes can work on the longest chain. But why would a node help other nodes by broadcasting to them the longest chain?

What if most of the nodes don't broadcast the longest chains in order to delay the mining of other nodes?

Source: What is the incentive for a bitcoin node to broadcast the newly received block as fast as possible?