As said, they want to buy when the price is the cheapest. Sometimes if you book a long time in advancr (eg 6 months) they will wait a few months before buying the ticket because their statistics say the ticket is cheapest 2 months before the flight. Occasionally it can mean they don't manage to book before the flight is full. Not all companies do that though, but many of the cheapest/shadiest do.
What reseller doesn't buy tickets when you book? They get paid when you buy, why would they refuse to buy?