It's not just mobile though - it's different dpi (4k screens are getting more popular), window sizes and zoom levels. Mobile is probably not a target for the forms apps the OP is talking about, but tablets may be, and different generations of various laptops and PCs are.
Web works across everything with little to no extra effort, whereas native app built with WYSIWYG UI builder is going to be constrained to certain hardware and take extra effort for handling display variations.