Also - yeah, the economics discussion about payments isn't always simple - but the general rule of economic discussion is "all else equal".
No, sorry, these are dependent variables / opportunity costs, we're talking about.
Externalities and opportunity costs to a decision are a very real thing, and they are important to consider in any economic decision. If you're
not taking these into account, even the ones that affect the decision maker themselves, and are completely known variables, then it's only a student-level textbook math puzzle that's not designed to model the real world. What you're saying is equivalent to a physics student saying "well the general rule of
physics discussion is that we treat all masses as points with no friction". That's correct that you can simplify thinking about things by assuming most of reality doesn't exist, but it's not a valid counter-argument against engineers taking those things into account.
If you get a home loan, you don't have to
pay rent, so any cost/benefit analysis of the decision to get a loan to buy a home is severely flawed if it doesn't take that into account (the rent payments are
opportunity cost).
If you get a car loan, you don't have to take whather transport options you did before you had the car, which needs to factor into the economic case. Also, if you sold your old car to get the new one then the money
not spent on things like needed repairs on the old car need to be factored into the economic decision, that's a dependent variable, because money that you're not spending
because of the decision under question is
not an "all else equal" variable: if your old car was kaput and needs $5000 of engine work to remain roadworthy, then clearly whatever
alternative choice you have to repairing it is $5000 more attractive.
If you get a car, you can also secure employment. That's not an independent variable, it's a dependent variable: the car isn't incidental in that. Or, if you're self-employed and having the new vehicle means you can expand your business, it would just be wrong economically to not consider that in the decision.
Also, as for saving up, "skill at saving up" is a relevant dependent variable. By locking into repayments, fiscal discipline is enforced where it's lacking. This is not an all-else-equal variable either: the amount you save by belt-tightening is dependent on the size of the repayments.