Downvoting and closing are different
Downvoting is not used to determine whether a question is “too broad, unclear or subject to opinion”. (Note that “unresearched” is not the same thing as “unclear”.) Those judgements are used for deciding whether a question should exist on the site, not what its vote score should be.
This question does not show any research effort; it is unclear or not useful
A question can be narrow, clear, and answerable only objectively and still be deemed “not useful” by voters.
The point of voting is to sort questions according to usefulness to other readers. Downvoting is just as important as upvoting, so the reputation required to downvote is not very high so that all voting is encouraged. The more voting occurs, the closer a question's score gets to being an accurate measure of its usefulness.
In practice people do still downvote questions they think off topic (maybe because they don't have the authority to cast close votes), but downvotes still don't determine whether a question is on topic or not… even if some voters wish they could.
Too broad, unclear or subject to opinion
Meanwhile, “too broad, unclear or subject to opinion” has to do with whether the community deems a question to be answerable in the Stack Exchange, vote-driven system we have. This is used to decide whether a question should be put on hold or not. This is completely separate from whether a question is considered good and useful!
- A question can be useful, but still too broad, opinion-based, or unclear. Questions that are popular (highly upvoted) are put on hold every day.
- A question can be useless, but narrow, clear, and objectively-answerable. There are many, many low-voted or downvoted questions which are still acceptably on-topic for SE sites. There are many questions across the network that have low or even negative scores, but which remain open because they're acceptable to ask here.
Downvoting and close-voting are completely independent of each other because their votes measure completely different things.
Experience needed for each is different
Those who judge whether a question is “too broad, unclear or subject to opinion” already require much more experience than others: 3k is required on a non-beta site to cast hold/close and reopen votes.
Meanwhile, downvoting is not considered to need much experience. We don't need people to have a good understanding of how SE works to judge whether a question is useful. Voting doesn't require understanding how SE works, it only requires having an interest in the subject of a site.