ELI5 REQUEST: State of the network growth

Hi,
I would like to understand this statement better:
"Ethereum's scaling bottleneck is not the time it takes to reach consensus but cost of verifying the chain. If the state of the network grows too fast, the chain becomes too expensive to verify."
Could someone give it a go in ELI5 terms? Why does ETH become too expensive to verify if the network state grows too fast?