I suppose that they felt that spending pages telling people what money is for might be a but unnecessary. A bit naive on their part perhaps...but I for one am glad they made the decision they did.
I think you've missed the point.
What is the purpose of money
from the point of view of gameplay? Eg given that gameplay doesn't generally produce the result that armour or weapons ever get damaged; and given that the amounts of money that the game tends to be asume will be recovered make other cost of living expenses trivial; what is the money
for, in the game, other than writing bigger and bigger numbers in a box on the PC sheet?
Classic D&D implicitly answers this question by (i) giving me rules about how my 9th-or-thereabout level PC can build a castle or tower or hideout or whatever, and (ii) giving me costs for doing so which are at least within a ballpark order of magnitude of the amonts of money the game will result in my PC collecting.
3E and 4e answer the question, in a different way, with their rules for the cost of magic items combined with their expectations (implict in 3E, express in 4e) about what sorts of items what levels of PC should have.
It's not like collecting money for no gameplay purpose is
just what you do when you play a FRPG. I mean, maybe that was how it looked c 1977, but the 40 intervening years make a bit of a difference.
And I don’t think lazy and pragmatic mean the same thing. Pragmatism, to me, seems to include consideration. “What’s the best use of our efforts?”
Laziness does not. “What’s the easiest way to get this done?”
There seems to be some fundamental confusion here.
Lazy, used of writing or composition or design, isn't a speculation about the motives of the creator. It's a description of what they have created - about the way the work innovates within its field, or is pastiche, or relies on cliches or tired tropes, or - in the case of RPG design - rests upon assumptions or undertanding of how the game will be played that aren't spelled out, or fails to address what might be anticipated as forseeable challenges or conflicts likely to arise in using the rules in play.
There can be reasons to use lazy writing - in cinema, for example, pastiche often seems to be more commercially popular than genuniely new work - and one can easily see the same being true in relation to RPG design. But that is a separate matter.
I guess I'll finish by saying I don't even have well-formed views about whether 5e evinces lazy design, except for the aforementioned bit about money. My horse in this particular race is the idea that there is such a thing as criticism of a work which is separate from either (i) speculating about the motives and merits of its creators, or (ii) collecting data about its commercial success. I'm pretty strogly committed to criciticsm in that sense being a thing.