Why do sellers do lots of 'random' bricks?
I think their rationale is simple: selling 10 smaller lots is bringing them more money than selling it all in one big lot. It might also be a way to mask the fact that there are very few good/interesting pieces in the lot; by splitting it up but using the full lot picture every purchaser hopes they might get the good pieces. Finally, there could be some deception going on ... if you see a picture with a lot of run-of-the-mill parts and some rare/expensive ones, and you buy the whole lot, you expect to get those pieces. If you buy a part of the lot, you have no way to validate that those special pieces were ever included (they could have gone to one of the other purchasers, or, and this is where the deception comes in, they might have never been included).

Edit: there might also be a more practical reason: cutting the lot up into certain sizes might allow the seller to ship using a favorable shipping rate (e.g. First Class Package in the US) versus a more expensive shipping method for the entire lot.