If you're a typical geek and into the coding and "setting up technology" aspects of a project, it's natural that adding the shine, doing design tweaks, thinking about marketing, and the "soft" elements is going to be less fun and, therefore, "harder."
I suspect if you took a print designer who was asked to design and then code a Web site, they'd spend a ton of time on the design and enjoy it, but then find the "10%" of time taken to try and turn it into HTML and upload it someplace would be similarly difficult.
Polishing takes time, because it's similar to design: You spent hours trying how different options look or ensuring that the use flow is flawless. There are lots of front-end coders who consider this as the fun part, they like making things look good.
People enjoying backend programming doesn't realize this, they just think that it is the trivial part of the programming (and thus requiring a little amount of time), although totally different skills are needed.
Also, many UI libraries are very cumbersome for this kind of microtweaking. CSS is very good for it, but you still encounter lots of problems when you try to make just small change.
I suspect if you took a print designer who was asked to design and then code a Web site, they'd spend a ton of time on the design and enjoy it, but then find the "10%" of time taken to try and turn it into HTML and upload it someplace would be similarly difficult.