Why blocks are validated instead of transactions? In most of the conventional blockchains, blocks are validated instead of transactions. Why cant' transactions validated directly?
The structure of basic blockchains are linked lists, by having DAGs, is it possible to have a graph-chain of transactions?
Some transactions cannot be fully validated outside of blocks.

If you use OP_CLTV, a transaction can be valid only after a certain block height, and not before ; if you don't know which block it's in, there's no way to know if it is valid or not.