tl;dr: because that’s what the market wants.
The commodity PC business is very competitive. The margin on a typical consumer desktop or laptop computer is at break-even or less. Mostly, profits come from selling support or extended warranties and the like.
Imagine you are a PC manufacturer. Due to intense pricing pressure, you are basically losing a couple dollars on every sale. Now imagine some software vendor comes along and offers you $10 per unit to preload Office, or some antivirus, or whatever. Would you rather:
- Say no, and continue to lose money on every sale.
- Say no, and raise prices to cover costs, but lose all sales to your competitor.
- Say yes, and preload the crapware to lower the cost-per-unit?
Hint: only one of these results in you staying in business.
How is any of this the fault of consumers? Because consumers will search online for five hours to save 7 bucks on an 600 dollar laptop. In this kind of environment, you either preload trialware, or your prices aren’t low enough to move any units.
The same kind of nonsense is also happening to the airline industry, which is why they are always inventing baggage fees and dreaming up ways to charge customers for using the restroom.
 To be fair, there are other causes, one of them being an entire industry designed to work when oil is < 80 dollars/barrel and go bankrupt when it isn’t.