Framing fine art is more tricky than framing a poster or a certificate. The artist usually will want to play an inclusive role in the choice. All will depend on what the artist determines is the message that they wish to convey with the art frame choice.
Artists choose plain wooden frames for an obvious reason. Framing fine art requires that the frame does not overpower an artwork, that is sits back and does not take part in the aesthetic experience optically.
Plain wooden frames especially sympathetic hard wood frames meet both of the above criteria. They are moldings that stand the test of time. Moldings are constantly in and out of fashion and discontinued for not selling well. Frames from only a few years ago often can no longer be sourced.
Plain wooden frames will be as relevant today as they will be in 100 years time. Always a safe choice, as wood has the character of compatibility with most artworks. Strange but true.