If you wish to be a co-founder, you typically have to be willing to take more than 40% pay cut for much longer. You must be willing to defer your paycheck so that your other employees (who are usually on a paycut too) can get paid if funds get tight - this is even expected of most CO employees in the company but the first ones to go are always the founder's. You are usually involved in most fundraising activity in addition to your other duties, whatever they may be* and customer support early on.
The list is quite long, I was once a highly entitled "engineer" until I built a few of my own companies. The dynamics are so much more complicated and tailored to each individual situation that it's disingenuous to lay out blanket statements the way most people in these comment threads have been doing.
Being a founder is brutal.
I'm not saying key engineers aren't entitled to good or fair treatment or that it is impossible to become a co-founder if you're #3 or #4 or #5, but I do not believe you're entitled to founder status just because you're an early hire. Sometimes it can happen that way if you choose to take on those responsibilities and risk and negotiate that dynamic with the other founders - setting up achievement milestones and what-not.
Similarly: Just because you're a founder doesn't mean you're actually entitled to the role of C*O (though you may hold that position early on as it needs to be "filled" it can quickly out-grow you). Some can become those roles and some simply aren't capable of scaling with the needs of the company.