You shouldn't be looking at the mempools but learn how mining works instead.
Bitcoin has a "difficulty" mechanism that ensures that the "target" is difficult enough for the network's total hashrate to mine a block at 10 minutes average.
To mine a block, miners are basically looking for a SHA256d hash that's lower than the target; but the number of miners are always changing as well as their hashrate,
so the target needs to be readjusted periodically to be less or more difficult based on the networks total hash rate.
Additionally, that hashing algorithm's result is unpredictable, so there's even a chance that the next block could be mined within the next second or the next hour.
(it happened quite a few times already)