Airlines usually don't like carrying more fuel than they have to, so they don't just "fill up" but carefully calculate what they need as dlombard says.

If you load more than you need, you just end up burning extra fuel to carry it with you. IIRC, the rate of burn is about 4% per hour - so if you load more fuel than you need at the beginning of a 10-hour flight, you'll basically end up throwing away almost half of that in additional fuel burn by the time you arrive.