Because a twin is much more likely to be in the same category and class of what your future employer will be paying you to fly.

You may have both engines operating but you are managing a complex aircraft as well as its multiple systems and adjustments and idiosyncrasies of keeping it all running in harmony.

Multi time is not necessarily vital. They need to have a cutoff somewhere and this industry is insurance driven. If they say multi time is a requirement then they know they are not going to get someone who has 1100 hours of only 152 time. As for the hours, 50, 100, 200, 500, 100 PIC etc, those are just arbitrary numbers chosen by the insurance companies or the current hiring market.

P.S. The Da42 is BARELY a twin...it has fadec.