so the key question here is what a "mid-level" engineer can do for your company early. An engineer with 5 years of experience at FAANG will easily make ~200-250k, competitive scaling startups and public companies will compensate equivalently withing ~30% in liquid compensation.
If that engineer instead works for 100k and takes 100k in illiquid equity which may or may not be worth something in 10 years, they are losing well over a million dollars in opportunity. Given that 5 more years at a top-tier firm would double their compensation they could easily be giving up multiple millions. To be employee #5-50 this individual should be getting an equity option that would reasonably be worth millions in an exit scenario for it to make sense.
Most startups simply aren't competitive for this mid-level engineer given current compensation practices. Which leads to a sub-selection of talent for those who for various reasons aren't willing or able to work at a firm willing to pay that level of compensation. Bear in mind, if the engineer had liquid equity from a public corporation they would be able to leave equity in hand after 4 years.
Whether this is a problem or not is an interesting question for startup CEOs. Generally I've found success in software is more closely correlated to the quality of employees than the quantity, and historically companies with generous compensation practices seem to be the successful ones.