preview.app has been around since NextSTEP, which used display postscript for its windowing engine. Having a first-class pdf reader falls out pretty naturally.
Sure. Spending money on implementing okayish PDF forms support.. I'm going to out on a limb here and say that it wasn't done out a position of strength.
wild guess here -- MacOS had given way to MacOSX, but commercial printing and publication workflows had not. Windows via market mechanisms (plus assorted dirty tricks), had a lot of installed desktops by that time in the print world, with a much smaller set of the artsy people and some others still championing Macs. When MacOSX got stable after the first few releases, the NeXTstep postscript handlers were there, and Apple wanted to compete to get users to buy new Macintosh hardware for print. Anything and everything to attract print people was definitely included.
Forms were appealing to some businesses and I dont have insights on that part of it, other than noting that odd media formats and bolt-on web'by things were being added to PDF at the time, and as for common forms solutions, apparently fifteen years later its still not settled.