Because I don't believe it's possible to accurately predict what sort of gains I'll make over a year, and trying to progress at a rate that's either too fast or too slow could mean not making gains that I could have otherwise.
Same things go for everyone else. I'm not saying this to give you a hard time, but your logic is really like "I don't want to put pressure on myself", which is usually a loser's mentality. And this is coming from somebody who's failed miserably two years in a row!
You can never make accurate predictions on your future gains, as they are affected by tons of things that are not 100% in your control. Ideally, the goals should be challenging but realistic and the idea is to help you build a general plan on how you're going to achieve them and maybe include shorter-term goals.
It's not like if you're making faster gains your goals are going to hold you back; goals are meant to be adjustable. If you achieve them sooner than expected you simply replace them with new goals (hell, DrB just had to replace his June DL goal and it's not even a week in the year!), and if, for whatever reason, you fail to reach them, well that's fucking life, you man up, reassess, adjust (short-term and long-term goals/timeframes) and keep trying. You still had a goal to work towards and assess your progress by.
Goal setting is a major part of sports psychology, btw. It's considered an important part of an athlete's plan pretty much across all competitive sports.
TL;DR: put some fucking numbers you consider challenging but realistically possible for you, and make a fucking plan (with shorter term goals/timeframes) to fucking get there.