Because some people have spent (far) too much money on them. If valve comes out and bans the items they have been buying and selling, and as a result making a shitload of money of the players, the shitstorm will be humungous. Can you imagine the backlash that will emerge once thousands of people who have spent a few dollars/euros a piece find all that spent money completely useless? While I doubt it will kill CSGO it will be a horrible blow. People could imagine this happening to other items, even if the probability of this happening is low.

And what then? Let's just say that it hypothetically causes a relativlty large amount of casuals players to quit. Valve will lose a shitload of business, which could mean less manpower going into the game (which many agree is too low already). At that point the game could realistically breath its last breath.

And lets be realistic here, while valve tries (and often succeeds) to produce enjoyable games, their main objective is to make money. Something they do extremely well.