It is because of the LBA and 32 bit counting you get with mbr.
In theory you can have a starting sector at 2 tib.
Then you have nowhere to go. The partition will have no size - it can't count any higher.
Anything above 2tib cannot be addressed using mbr
It might be better to think of it as a start sector anywhere up to 2 tib and a max disk size of 2tib.
Gpt uses 64 bit counting - that is how you can get bigger disks recognised.