Why?
Discussion
Because a partial node can not validate all transactions and blocks, obviously. If you do not validate yourself, then you are trusting someone else to validate for you.
Don't trust, verify.
Yes, but not blocks of history, new blocks. Can they verify those?
A partial node can validate the block header but it cannot validate the transactions in the block.
And a pruned node?
yes a pruned node validates all blocks and all transactions. It is fully trustless. But because it is pruned it is not appropriate for something like a block explorer which may seek to view transactions and blocks that have been pruned.
But yes, pruned nodes are trustless.