Really?
I'd rather contribute to something that is actually going to be successful.
If you say you need £1000000 to go to space, and ask for £10 sponsorship, if you tell me you've so far raised nothing, what chance is there of my money going to use as intended? If on the other hand you've raised £999000 I can be more confident you are actually going to reach your goal.
Depends on whether you view your donation as a good will gesture/reimbursement of sorts for services rendered – or if you consider in an investment with more clearly defined objectives at the end of the fundraising campaign.
I suspect it's just basic human behaviour. If other people are doing it, then it's somewhat normalised.
Also personally, it seems to me that if you have a goal, then that's a minimum. If you don't reach it, the thing doesn't happen. A website never reaching its funding goals is one that seems more likely to shut down, and what's the point of supporting something that could shut down tomorrow because it can't pay it's way. I know that's self reinforcing but there it is.
Cynically, nobody is going to donate to a full or nearly full "donations requested" bar, so nobody is going to specify a bar that gets full.