Because spam is CHEAP... it really doesn't cost anything to spam... and if only 1 out of 1000 attempts is successful, that more than pays for the cost.
Success being measured by someone using a link in the spam to access the advertisers website?
But even if doing the spam is cheap, setting up a spam server, running the software, monitoring the success rate, the accounting and overhead every business has, why is it gibberish with some links in it? Why not something more likely to get a hit. In the Coach spam on XenForo, why not a good ad more likely to get a hit vs. the gibberish. Cost of pasting a good ad vs. pasting gibberish is the same.
And don't the links in the spam must have some identifier for the spammer to get paid for the hits his spam creates?
Which gets to the money trail back to the advertiser paying for the spam? Spammer needs some ID in the spam to get paid for it's success so the spammer is ID'd to his bank account. Advertiser is tracking the hits and the spam source for the hits and paying into the spammer's account. No different than tracking down drug dealers and terrorists via the money the trail.