Unfortunately software companies now view customisability as a bad thing, as their priority is profit, and that often involves making the client less usable. Discord wouldn't be able to periodically shift around the UI to make you accidentally click on the "Gift Nitro" button if they had to respect the layout you customised.

Why would moving the UI about result in 'accidental revenue'? We're not talking about ads here.

I'm just as confused as you are as to why anyone on the planet would click where the "attach image" button used to be, see a popup for gifting Nitro, and go "sure, let's do that instead", but evidently Discord thinks that at least some people are going to do that, or else they wouldn't have made such an absurdly hostile change in the first place.

Presumably they're banking on people who would have gifted Nitro in the first place but were just unaware of the feature.

I wish I could choose my own font!