There are really three questions here:
1. What are the absolute minimum time deadlines for checking luggage at the counter and presenting yourself at the gate?
2. How much time do you want to allow for unforeseen delays, such as a delayed hotel shuttle departure, traffic delays between the hotel and airport, long lines at the counter, long lines at the TSA checkpoint, or other surprises?
3. How much time do you want for an unhurried breakfast in the terminal concourse where your gate is?
If flying United Airlines out of Newark, NJ (EWR), check-in and baggage check must be completed 45 minutes before departure time (regardless of how long the line is), and you must be available for boarding at the gate 15 minutes before departure time (regardless of how long the TSA process takes).
Nobody NEEDS to be at the airport 2 hours early for a domestic flight, even at the biggest and busiest airports in the United States. However, many of us try to be at the airport 2 hours early because that allows for relaxed, relatively stress-free travel.
Also, boarding begins 30 minutes before departure, so 2 hours really means 90 minutes before boarding.