This is a curse of web design - you have to guess if it's clickable. You're not allowed to have boxes, or lines, or raised buttons. It's almost a dark pattern. It seems to be a holdover from mobile, where unmarked buttons were a thing, but the buttons were bigger and the UIs simpler. Now they've migrated to busier interfaces and become smaller.

Date pickers are terrible if not near the current date. Ones for long-term data sets where you set the start and end date are a huge pain. Like getting inflation data starting in 1960, or coronavirus data starting in 2019.