Why do power banks have such a limited amperage vs. 18650 battery packs?
Because those 'power banks' with 5V USB outputs are intended to be used to charge things like phones, which are only supposed to draw a limited amount of current.  

Also, its more expensive &amp; takes more space to make the 5V regulator circuit for higher currents, so why would the manufacturer's do that when high currents fall outside the intended use case for the device?

The point of having large-capacity batteries in these power banks is to provide larger energy storage capacity over time, not more instantaneous current.