Why does MetaMask need an internet connection to input/validate a seed?

Yesterday I installed MetaMask to Brave and selected the input wallet option.

I then proceeded to take my paper backup with my seed. When I was about to input my seed, took the Ethernet cable from my PC to disconnect from the internet.

I wrote my seed but when I hit continue MetaMask was throwing an error. I re-typed the seed, this time with commas in between each word and still got the error.

I was thinking, there's no way this thing needs internet in order to validate my seed, right? Well, I re-connected the Ethernet cable and then 3 seconds later tried to validate once again and… success! (???).

Can someone explain to me, why does MetaMask need an active internet connection in order to validate my seed? Why can't I use MetaMask at least in a semi-offline environment?