The executive exists to enforce the law the legislature writes primarily to make sure the legislature isn't in charge of enforcing the laws. It's a check on the power of the legislature.
You could still have discretion with the legislature in charge of executing on their own laws. I think countries exist like that, but I don't know enough to say which.
A separate executive is not necessary to have discretion or pardons or flexibility in law. A separate executive is necessary if you want physically different human beings controlling the organizations who enforce the law (DOJ, FBI, etc)
Consider that Judges and the judicial branch of the government ALSO gets to use subjectivity and their own opinion in adjudicating cases. Another check.
The entire point of the Constitution was to put the power of a King in a bunch of different hands, and then tie some of those hands with specific constraints, and then give a couple different options on how to change those constraints over time. Leeway and discretion goes both ways, so Congress does have the ability to further constrain such discretion. A previous president tried to argue he could choose to not spend money congress told him to spend, so they wrote up a bill saying very clearly, Uh, no, if we say spend, you spend. They have that power as a check on the power of the executive. All three branches are ostensibly MEANT to be vying for power. It's an antagonistic system, like the court system. The founders loved that shit. In reality, it probably is a dysfunctional system that modern systems engineers would not like, and other countries get that "system fights and moderates itself" effect by encouraging coalitions between parties in a strong parliament. IMO those have demonstrated better stability. I'm not convinced the US would have survived getting it's whole shit blown up like the UK did.
Checks. And. Balances. 5th grade civics class.