Hello,
do you think that the feature of import private keys from wallet.dat would be appreciated by users? Export keys from Bitcoin Core is not noob-proof
Bitcoin Core wallet.dat has hundreds of hidden addresses in a "pool" of addresses.
Importing the entire wallet would take a long time, and could swamp your Electrum wallet with keys that are not able to be backed up by your seed phrase.
This is why it is better for the user to send their bitcoins to a newly created Electrum wallet.
A "noob" type person would probably think that "oh, ok, now I have imported my wallet.dat, I can just remember this 12 word phrase and delete my wallet, right? *deletes*"
No one wants something like that to happen, so just send.