Because of game mechanics. If game is designed for a 3rd person view it will not be well playable in 1st person. I dont mean its not possble, no, its just a camera hack, but all UI markers, all battle mechanics, interaction... everything will be not right. Plus players in 3rd view will get serious advantage in battles against 1st view players and so on.
There is only one right decision : make both, let players choose. Ark 1, Arma etc... Restricting view selection also lets players to decide what game expirience they want - more dynamic arcade/souls like or more hardcore realistic.