Sony, Universal, BMG, EMI and Warner (and their innumerable subsidiaries) account for over 90% of the music sold in the United States and up to 80% globally. The majors are typically characterised as greedy, cynical, bandwagon-jumping and litigious (see also RIAA).
Not all artists signing to major labels suck, but the majority do fall into one of two categories:
(1) those plucked from obscurity, who are given some songs, handed over to a stylist and aggressively marketed
(2) those with previous careers and original material, who agree to remove all traces of innovation from their music, are rejected by their old fans as sellouts and are aggressively marketed.
Regardless of their popularity, most will be financially crippled by the label's dubious accounting and will be dropped the moment sales start to falter.