[AgentME]

For almost anything more interactive than a static document, modularizing UI code primarily based on component boundaries makes a lot more sense than modularizing primarily based on markup / interactivity / styling boundaries.

[qbasic_forever]

Exactly, it comes from a bygone era when there was a clear separation between frontend developers (i.e. people that wrote markup, CSS, and jquery style JS) and backend developers (people that wrote PHP, ASP, java, etc. to spit out markup that frontend devs wrote). Nowadays everyone is basically full stack with specializations or focus in specific parts.