Because it was already well-tested and well-bypassed by any possible means on FOnline: 2238, and we don't want to make the same mistakes. Sandboxie, multilaunchers, virtual machines, another laptop/PC, connected via different provider, proxy, VPN, etc.
Also it has been already discussed 238183 times especially after a new player comes and says "well it would be cool if you could only play ONE char because of all possible abuse with many chars" (just use search), but it's not going to happen, period.