This situation is a textbook example of a tragedy of the commons. Each PC manufacturer stands to gain from including bloatware. Consumers generally don't decide between different PC makers based on the amount of bloatware installed (because they all have it), so the cost is practically nothing, while the direct, short-term financial benefit is substantial.
However, consumers *do* consider subjective frustration when choosing a computing platform, and bloatware increases the frustration consumers feel toward the PC platform as a whole, reducing its market share. In short, bloatware is like pollution: nobody pays to dump sewage in the river, but everyone gets sick.
There are two common solutions to the commons problem: regulation, which isn't really feasible in a private market like that for PCs, or property rights that give actors an incentive to maintain the commons. The latter tactic explains why walled gardens have grown so explosively: their "owners" (Apple, Google, and to some extent, mobile carriers) have a strong incentive to not pollute the market.