Are you seriously asking why Apple offers its users a built-in app to view the photos taken with their phone? You'd prefer if users had to choose and install a photo management app before they could take a picture?
The burden of proof is on the people requesting the feature. The reason for not building any feature is simple: users don't care about it and there are more valuable other features to build. (As evidenced by system apps not being deletable on iOS for its first decade.)
> The burden of proof is on the people requesting the feature.
Fortunately there are other reasons why a business may be forced to do something, and not allowed to be the sole judge/jury/executioner.
One of those is anti-trust action - something that greatly benefits humanity as a whole, even though the alternative may (or may not) be cheaper for the business.
Other examples are employee protections, consumer protections, constitutional amendments etc.
Which goes back to my original point: there is literally no benefit besides navel gazing for requiring the photos app to be deletable. Just because there are many reasons one could compel a company to do something does not mean any of them are applicable to this request.
