That doesn't really make any sense to do that though. Sure the node will not have a way to verify that mempool transactions are valid, while it is doing IBD, but what is stopping it from just collecting the top few vMB (like the default 300vMB) of transactions, and then only verify those transactions after the IBD is done and discard or keep them accordingly?
It requires a lot of developers' work to write a ton of code handling these cases and rewriting a lot of the logic that makes your node work. For example when you receive a tx, it is handled as a peer sending you that tx and that peer is going to be evaluated so that you can "ban" it in case it was malicious and were sending you invalid txs. The devs have to re-write the code to handle this case and skip the peer score logic.
They'll also have to write additional logic to skip a bunch of your mempool preference such as minimum fee since your node has no idea what fee they're paying (UTXO set not yet up-to-date).
It's just easier to ignore the tx when your node is during IBD.
