what on earth would you want vegetables on a burger for
Lettuce can be nice, as long as restraint is shown and they don't give you those crunchy white bits.
Onion can add nice flavour, depending on the variety of onion and how it's prepared. (Sometimes it's absolutely acrid and terrible.) Same with pickles.
Tomato is a straight up NOPE. The texture just destroys everything there is to love about a burger.