Founders often have slightly higher market value (though not always) than first employees, so they are giving up more to go the startup route.
Separately, TFA further underestimates founder risk as they are typically not taking salary during pre-seed, and no or low salary during seed. However employees 1-5 typically get mostly cash, often much closer to market.
Thirdly, there is also often a lot more stress in being the founder. It is a complex, all day job. You have the weight of keeping things going for all employees, and when cash is low it’s your paycheck that gets delayed/cut first, not your employees.
That said I am all for reasonable early stage liquidity where it makes sense, but as many other commenters have mentioned, it tends to not be life changing super early for most early employees. Most employees would rather keep the bet on the table. Also, I am strongly against large founder secondaries. I think it’s helpful for founder to remain feeling “not financially successful”, especially first time founders, so that they keep their heart in the game. I followed this practice with my companies.