I agree with you, that if a question is worth answering then it is also worth up voting.
When questions have 2 or 3 answers each with up-votes but the question itself is sat on 0 votes, I think it could become disheartening for those who have asked the question to engage further with our site and community.
For questions, the up-vote button is titled "This question shows research effort; it is useful and clear".
Consider (made up examples):
"I read that …" [Where did you read it?]
"I read that …" [But exactly what did it say?]
"T͟h͟i͟s͟ says that …" [The link is to a textbook; are we expected to read it all?]