Prep is not "play" as I would normally use the word. Let's not get into an "are tacos sandwiches" discussion--the purpose of words is to help us categorize things, we
select those categories, not objectively enforce them from on high.
Certain aspects of prep work are genuinely enjoyable in their own right. Speculating about where things might go, trying to devise monsters that are interesting and engaging, trying to solve a conundrum a player has dropped into your lap, etc. And I have zero problems with saying that prep can be a form of pure entertainment all on its own, rather than mere "work" one must do. Prep can be
fun.
But "fun" is not the same as "play." Almost anything can be fun--even legitimate, actual
work can be fun (and that's a lovely thing, when you can find it). For me, though, "play" requires a bit more rules, albeit "rules" understood in a relatively loose sense. Prep work doesn't really have rules, because you can do more or less whatever you want as long as you do the effort to make it work. It's pretty close to unrestrained creative freedom.
You mentioned "it doesn't have a win condition, but neither does D&D," and what I'd say there is that you've misidentified
one common form of the (soft-definition) "rules" that turn things into play as the ONLY form of "rules" that work. A win condition is great because it's very
simple, and thus enables a wide variety of forms of play (e.g. when animals play with one another, the common win condition of "hold the dominant position" is the "rule" that makes it into actual play--but note how incredibly loose and flexible that "rule" is, hence my repeated use of quotation marks on it.) When you're at the table playing D&D, you don't have unrestrained creative freedom, because there are both hard
rule rules and soft
best-practices "rules" that heavily constrain your behavior. (Of course, the fact that I think this applies to the DM is a separate and EXTREMELY TEDIOUS argument I really,
really don't want to have, so if you disagree, please let us NOT drag that into this thread too.)
As a different example of "rules" that don't strictly involve "winning," consider things like a group engaging in improvisational jazz music. I would absolutely consider a group of musicians showing off their skills to be "playing" (and not just in the pedantic sense of "playing music," I mean engaging in game-like behavior), even though there's no real win condition besides getting the most admiration from your peers. Instead, the point of play is to show off technique, and it is that very technique which forms the "rules" of this kind of play. Hanging out with your buds and cracking jokes is another example, where there is no real "win" condition, but there is a goal (elicit lots of laughter) and certain "rules" (acceptable forms of comedy, puns are
terrible terrific, whether impressions or references qualify, etc.)
I 110% believe that you have a TON of fun doing at least some kinds of prep work. But I'm not really convinced that that prep work counts as "play" in the way most people would consider "play" to work, in much the same way that
if you asked someone to give you that sandwich, they would be very confused if what you meant was the hot dog in front of them.
