I feel strange comming to the defense of junk mail, but here goes.
If there was not junk mail, your stamps would be higher than $0.34, not lower. The post office makes more profit delivering junk mail than it does delivering first class mail. Junk mail actually subsidises your personal letter, not the other way around.
The rates are lower for junk mail because the mailer has taken steps that save the post office money.
Junk mail also helps the post office cover their fixed costs. Your mail carrier has to stop and check every mailbox on thier route, even if they have no mail for it. That's a fixed cost for the post office. The marginal cost of having the carrier stick a bundle of junk mail in your mailbox is much less than the postage on those letters.
The same is true of the mail network. The post office has to maintain a network of post offices and transfer letters between them. The marginal cost of transfering some additonal junk mail through this already exising network is not as much as the postage for those letters.
Junk mail may be a PITA, but it is not the reason your first class stamps are so high.