Because Mew is an event legendary.
Event Legendaries are generally extremely rare, as they are only available via a limited-time event. There are two exceptions to this: Mew, who can be caught in Emerald, and Deoxys, who was event-exclusive up until ORAS, where it could be caught in the Delta Episode. Events for these legendaries are few and far between; there has been only one Keldeo event, two Genesect events, one Arceus event, etc.
The only way to get Mew in multiple games is to Pal Park it from Emerald to DPPt, then send it to BW/BW2, then Poké Transfer it to XY/ORAS. You could also hope beyond hope for a lucky Wonder Trade (it's not impossible, one user on the DB got a Diancie from Wonder Trade), hack, or play Emerald over and over and keep transferring lots of Mews.
If it makes you feel better, catch a bunch of Ditto. There's a theory that Ditto was another attempt t oclone Mew, but failed and resulted in a pinkish-purplish blob. So technically, whenecer you have a bunch of Ditto, you have a bunch of Mews!