Why take on so many extra laws when it doesn't gain you anything?

While this is one of the reasons, it is actually secondary, in that it considers what is best for the, as yet non-Jewish, potential convert. What is MUCH more important from the communal Jewish POV, is to ensure that the potential convert is very sincere in their wish to be Jewish, and fully observant as such. As I explained directly above the comment you responded to, Judaism has a "group contract" with G-d, so before "allowing" anybody to "join the group", we will want to ensure that they intend [to the best of their abilities], to uphold their "contractual obligations". A Jew that sins, reflects badly on ALL other Jews as well. So why ask for trouble, and bring in insincere converts! This is why, BTW, that Orthodox Jews in principal reject as invalid, [almost all] conversions done by non-Orthodox Movements. Because these movements as such, do not observe the Law as we see it, let alone require this of their "converts" or educate them properly with regard to the Law.

A more mundane parallel, would the Bar of Law requiring potential Lawyers, to pass an Ethics exam, before being allowed to "practice law". The Bar [in theory] doesn't want the legal profession to be disgraced...