Your body can synthesize creatine, so it's not essential, but meat is a source of exogenous creatine and eating it results in higher stored creatine levels, which is slightly good for muscle power endurance in the ~5-15 second effort range. Supplementing produces slightly higher relative improvements in creatine levels in vegetarians and vegans than it does in people who eat meat.

Because they lack all kinds of substances that meateaters get plenty of.

Nobody gets fired for supplementing vegans.

Because meat is the best source of creatine from diet.

Well, if vegetarians and those of the Vegan religion don't get enough creatine it is most likely related to them not partaking of an important source of the substance: meat.

Not vouching for creatine, but in general, humans get some vital nutrients from animal consumption. Vegans have to take some B-vitamins for best health due to what they don't get from meat. I assume creatine is the same.