95% of builds on Path of Exile scale with special uniques or powerful rares
The reason it is still fun after 20+ hours of play is because of the skill and gear progression that comes with most builds in the game. 95% of builds on PoE scale with special uniques or powerful rares as well as getting gear that has more linked sockets (5 socketed and linked sockets on gear are a lot more rare than 4 linked gear, and 6 linked gear extremely rare and endgame to acheive in most cases). Both of these will change will change how the skill works/increase their damage output significantly, making you want to progress even more to see how far you can take a build idea.
So the long and short of it is yes in it's most basic form the fun of playing the same skill over a long period is to see the returns of your investment into the skill and seeing how far you can take it, which is much more rewarding than it sounds due to the sheer complexity and number of options the game has to offer.

May
Path of Exile Skill Revamps - Part 6
It's only 3 days to the releasing of POE Incursion League, which is the core content of 3.3.0 updates. And GGG's Skill Revamps has proceeded to part 6, so I'd like to show you more details about Skill Revamps - Part 6

Dec
Most players between an eternal and a stack of exalts
crafting will be far far more about finding an already good multi t1 base and throwing an exalt at it than it will be about crafting 6T1's that you want from a base white item

Sep
Path of Exile 3.12 Heist Preview
A preview of the next season of Path of Exile has been released, called Heist. PoE 3.12 Heist will add a lot of content to the game, including npcs and brand new items. Let us know the latest information of PoE 3.12!

Mar
poe4orbs:PoE 3.10 Delirium Duelist Popular Starter Build
PoE 3.10 has arrived! Are you ready to take on new challenges? Take a look at the PoE 3.10 Delirium Duelist Builds(Champion, Gladiator, Slayer) prepared by R4pg for you!

Oct
Path of Exile Patch 2.4.0h Updated
Enabled the microtransactions from the Darkness Path of Exile Mystery Box: The Gloom Armour Set, the Obsidian Seraph Armour Set, the Gloom Wings and Gloom Herald Skill Effect can now be purchased

Dec
Path of Exile Useful Tips 10 - How Prevent Others From Using Portals In Your Hideout
Path of Exile Useful Tips 10 - How Prevent Others From Using Portals In Your Hideout