Historically, it hasn't been needed as the US doesn't need to stop people from leaving. Unlike some other countries, the US doesn't fine or imprison overstayers, so there's no reason to stop an overstayer from leaving (the penalty for an overstayer caught in the US is deportation, and if they're leaving they're already doing the same thing) as long as the US knows that they've overstayed so they can take it into account the next time the person comes to the US. (But even if the US doesn't know when exactly a person left, when in doubt, the burden is on the person to prove they left on time.)
And the other main reason is that it's very expensive to implement it now. At land border crossings, implementing exit checks means having to double the manpower and infrastructure at all border crossings (currently there is only facilities on the entering side; they will need to have it on both the entering and leaving side), which is very expensive given the volume of traffic across land borders.
At airports, it also requires re-configuring airports because having exit checks means that there must be an "international waiting area" for people who have passed exit checks that is separate from the domestic waiting area, but many airports currently use the same gates for both domestic and international departures. And it requires finding the space to house the exit checks (whereas right now all departing passengers have to go through is security checks); having to add immigration exit checks in addition to security checks would require a lot more space that might be difficult to find in some airport terminals.
So basically it is a ton of additional cost, for unclear benefit.