I'll elaborate. You'd think that by putting N powerbanks daisy chained together (except i imagine at either end) you get ~N times the mAh or runtime but i think you get probably .25N or less. the "last" powerbank in the chain, the one you'd charge to charge them all, would run out of power first, and about 15% of seconds more afterward, the second one, then the next, with the "first" powerbank, the one you're using for its USB ports to power a load lasting about 2x as long, no matter how many powerbanks you put in a row.
this is a supposition, but i don't think the numbers are very far off. Most powerbanks are 18650 or 26650 inside - flatter or "better" ones are lifepo or lipoly or whatever, instead of cylindrical Li-Ion. anyhow those are 4.2V nominal and USB wants 5V so all single cell or parallel'd 18650/26650 power banks are going to use a boost converter to get the voltage to 5.1V nominal at the USB terminal (assuming a dumb power bank, a power bank with power delivery will boost that even higher but also probably has multiple batteries in series, but that doesn't matter, it makes efficiency worse for our daisy chain regardless!). So these boost circuits "charge" something and store it until there's the correct number of electrons to equal whatever charge/joules is required to run the load. There is >15% wasted as heat either in the capacitor or the inductor (depending on the style of boost converter). There's another ~15% or so lost in the charging circuit, as it has to take 5.0VDC in and run a charging circuit (similar to mppt) on the battery at different voltages and/or amperages which again waste is generated as heat. Cable losses in the daisy chain probably account for a percent or two each, the indicator lights across all the power banks, plus all the microprocessors/etc inside of them probably waste another couple percent.
You'd lose a lot in efficiency. A power bank designed for this would have 2 cables and 1 or 2 wires, power transfer cables and sense/control wires. My engineering brain says "don't do this it's stupid and I think OP was making a joke I don't get"
