Why do my fingers lock in a straight position?

When we get older the alignment of fingers can change a bit as part of aging. If there is no pain or obvious deformity do not worry about it. If yes see your doc for a clinical exam.

Trigger finger is likely and swelling and inflammation around the tendon is most commonly the cause. Seeing your doc in the office will be best as to having a good plan going forward that may include occupational therapy, topicals, injections if needed.