Because, early on in Starcraft you'll probably be either playing against the computer, or friends of yours- and friends might go easy on you. Later, though, people are likely to be much more serious about it, and you'll be playing against those guys who can crush you in ten seconds flat (like Asian competition players.) For Halo, you usually get put into matches by rank, and by the time you make it to a high rank you can usually fight well enough against others.