Its not different developers but different services. Developers are competent. Its the product owners that are to blame. For example a hotel website, will have one minified concatenated script file, but then on top there might be other services, for example client side A/B testing. Which cant even be defer loaded and usually comes first! Sometimes analytics would come separate etc Other services show things like 'x number of people viewing the booking' etc etc

(BTW i don't see how you can have separate files, using modules or require etc would prove difficult to have split files, rather than one)