If any of the perfumes “released by celebrities” were simply under a designer’s name, of course they wouldn't get the flack that they do! While yes, they are in many cases a simple way to rake in more money and a long term cash flow for the celebrity, I can justify it as I think this is one of the most creative and fun ways to do it.
While most perfumes can roughly be attributed to an era, decade or trend, celebrity perfumes, particularly those from the 2000s such as the Britney Spears range, have a unique ability to smell EXACTLY like the late 2000s. Perhaps part of it is just because of the popularity and how common it was to smell people wearing it in the 2000s, but the perfumes are scents that often perfectly encapsulate the popular culture of the time - from the sickly sweet girly scents, to the blinged up designs of the bottles themselves, it’s pretty cool to have an encapsulation of the 2000s fashion and culture within a bottle.
Equally, as funny as it is to think about, I'm sure there will be communities of kids in a few decades dressing in "2020s" fashion and desperate to get their hands on Ariana Grande perfume, as some people are now with y2k. Not only do the perfumes have the time-centric appeal, but they can also be attributed to particular eras of a pop star and the various points of their career, collectable alongside albums or clothing - not to mention the closer connection it can give fans with the star themselves. While they are silly, are they any less silly than a T-shirt?