If there is no reason TO use an
Ambassador, logically speaking there is also no reason NOT to use one.
Every new model costs money to build - the artists' time and resources going to X takes away from their time and resources to do Y. That's probably also part of why they worked with the scans of physical models and didn't focus on other designs without a dedicated model or whose model had been damaged, something that gave them a baseline to build from, with the scale and proportions and details already something that they had references for, rather than build the whole thing from the ground up in the system.
Like there's the old tale of the Norway class, from First Contact - it was one of the designs created for that movie in CGI, the files corrupted between then and production of the various fleets in DS9, so it never appeared again, even though the other Starfleet ship designs from the movie did.
Sure, IDEALLY, they'd do this for every ship type, but when working with the time and budgets of a TV show, looking to put out weekly episodes, everything takes time and money, and you save both where you can. If the price of creating an Ambassador class model in CGI from scratch is (hypothetical easy to digest numbers incoming) $10000, and you can get both the Excelsior and Galaxy classes made by using their physical models as a baseline reference for about $12000 together, then that's the better investment.
This is why, for instance, we ended up with the infamous copy-paste fleet in Picard Season 1's finale, and why "all of the fleet" at Frontier Day was made up primarily of ship designs we'd seen at various points throughout the series, even though there was no reason not to expect a variety of Dominion War-era designs still around and in service a mere twenty-five years afterwards - every new design costs production money.
There's not really a good explanation for it in-universe, though one of the early DS9 novels did put out an idea that Bajor was allowed to build an Ambassador class as part of their efforts to get their own ship-building efforts back on their feet. Presumably, one could lean to the idea that the class/design had only a limited production run, and, by the time that Starfleet was in a position to build them on a larger scale, the Galaxy class design was already being finalized.