Well, on rare sunny winter days, these trees are visited by large solitary Anthophora bees. Each bee can visit a single tree hundreds of times. But even with these visits of pollinators, only a tiny fraction (5 - 10%) of flowers set fruit, which seems like a huge waste of flowers. Why such a waste?
The answer is probably due to the difference between males and females (I'm talking about trees here, but the case could be made for humans as well). Males seek different paths of reproductive success than females. The male parts of the almond trees (which produce the pollen) benefit from each bee visit as more and more pollen is spread to other trees. The huge number of flowers is a very attractive advertisement for the bees to come get pollen. The larger the advertisement, the better chances of male success. The large cover of thousands flowers attracts many more bees that carry pollen away, which increases the chances of male success.
The female parts of the tree (which is the style and ovules), on the other hand, enjoys the privilege of "choosing" the best pollen grains and being picky. Only the most fit male will make it to the ovule and produce a fruit, otherwise an abortion will be its fate. So only a twentieth of flowers set fruits, but these will be the best kids for mom!