It’s for 21 seconds only. So this is still way faster than Bitcoin’s 10 minute or something. But I agree, it’s a bit less secure maybe, but brings more speed. Still though, there are quite some safeties built in like Transaction as Proof Of Stake where you confirm a recent block on a valid chain as a user with every transaction.
Transaction as Proof of Stake (TaPoS)
The EOS.IO software requires every transaction to include the hash of a recent block header. This hash serves two purposes:
prevents a replay of a transaction on forks that do not include the referenced block; and
signals the network that a particular user and their stake are on a specific fork.
Over time all users end up directly confirming the blockchain which makes it difficult to forge counterfeit chains as the counterfeit would not be able to migrate transactions from the legitimate chain.
Looks like they try to find the right point between speeds and security. Same with switching to certain regions of the world by votes to keep the speeds high.