Why did you say that not miners hold these transactions? Anyone in the network, who have to mine, have to hold it, if I'm not mistaken. Cause miner just takes some transactions from a file and puts it in a new block. Nodes can be far away from each other and it can take some time to get new transactions from another node. (Longer, than just to read them from a file)
And in here https://www.politico.com/newsletters/digital-future-daily/2022/06/21/inside-the-mempool-where-crypto-risks-hide-00041132 it's said: "“Mem” is short for memory, and the mempool is where each network node keeps a list of spending crypto transactions."
If I'm mistaken, correct me please