Why not buy tomatoes at the current price? Because even if the price is going to go up in the long term, in the short term the price is likely to fluctuate, and if you're patient, you can probably buy them cheaper on sale.
Same with bitcoins, if you're not in a hurry to buy, you may be able to buy them cheaper from someone who is in a hurry to sell.
Ah ok, kinda makes some sense. Cheers