Because they seem to be very wrong, at least according to wikipedia (whom I would say is a trusty source on this, since they seem in line with other info pages).
I know wiki doesn't make a distinction between engines and that could explain the difference in performance, but take for example a Fokker 100, which was only offered with one engine option (according to the game, anyway). First of all, it's name is Fokker 100, not F100, but that's minor. Now, comparing numbers, they all look wrong. Min takeoff run, the game says the plane needs a 3508 ft runway, wiki and other sources say 4988. Range, 1448m in game vs 1522 elsewhere. Furthermore, according to wiki the 100 was actually offered with two engine options, both really similar tough. Both Rolls Royce Tay, but one 620-15 and another 650-15. Some specifications the game says correspond to the 650 version (thrust, for example, at 15100 lbf), but the game only list the 620 version, whose numbers are all wrong.
This is just one example, I can list loads (range on 747SP, min runway length on 777s) but I don't want to bore you. So, where do the numbers the game give us come from? Am I just looking in the wrong places, or maybe different units are used? I don't think that's the case, but you never know.