You can always decline, but the issue is that if you agree first and then realize that you won't be able to provide helpful feedback, then the manuscript just sits there and the editor has to find a new reviewer which can take time. So it could be unfair for the authors, to some degree.
Generally, one should have a good sense after reading the abstract whether ones background is sufficient to review it (though I had one or two bait-and-switch manuscripts).
Yes normally reviewers are folks who successfully publish articles. But the fact that you have been submitting might why editors have your contact info. It is still unusual and probably points to the changing publishing landscape (and some quality issues in science in general- though not sure how much is real and how much is just part of getting old).