You probably hold a single directional button for large portions of countless games, including revered ones like BotW, Witcher 3, RE4, HL2 and so on. Hell you probably spend more time pushing a single direction or bashing a single button in BotW, W3 and other open world games, than you do an entire play through of any Uncharted game.
Platforming is fun in Uncharted for the same reason as general traversing is in many games (despite the simplicity of both), for its sense of discovery, exploration, progression, visual or vista rewards and so on. Not all gameplay has to be challenging to be fun.
Also, of course you're entitled to your opinion, but the notion that Uncharted has weak gameplay seems so misplaced to me. It has some of the best third person shooting mobility and encounter design of any third person shooter out there.
Uncharteds dynamic level design and tactical diversity, especially with more potential for mobility and verticality, better set piece design and thrill factor, and what were to me satisfying weapons and solid AI, all made for highly compelling overall gameplay.
Heck, levels like these examples below were just breathtakingly well designed and well beyond anything in most third person shooters. And I'm not even getting in to Uncharted 4 or Lost Legacy, which both improved combat in a big way.
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1. Despite so many not liking this level, imo it is one of the of the best combat arenas ever designed in a third person shooter. So many amazing things about this. Procedural sea physics based moving platforms that also act as cover and change the inertia of your aiming when stood on, forcing added skill. Non stop pressure from enemies to stay mobile and mix things up. All kinds of cover options, high and low and everything in between (platforms, boats, debris, structures, ship innards, crates, boat masts etc), even diving inside the water itself. Stealth options, all sorts of approach options, clever level design and platforming, excellent AI etc etc.
http://youtu.be/xdNs6FqYHs4
2. Just an insanely well executed entire set piece segment. Lots of gameplay range, a constantly dynamic moving sequence with no apparent scenery repetition, that adds in platforming, stealth, boss fights (with both NPC's and the helicopter), changes in scenery (from jungle to snowy mountains), destruction and all sorts. The entire time there's the added challenge of having to compensate your aim because the train is moving, and so on.
Compare this train segment to that of any other similar segment in any other game (Gears of War, Killzone 2 etc) and it just shows you how much better ND are compared to those other developers.
http://youtu.be/eHMSyVxbOVc
http://youtu.be/TRDTO4heIIM
http://youtu.be/sxLQ4l_fsgY
http://youtu.be/NlAd2XOQdEs
3. Excellent, clever and unique use of changing, vertical airborne cover to add tactical options to the scenario.
http://www.youtube.com/watch?v=87Sd3qL26mQ
4. Another good example of excellent tactical diversity, and mobility options, even in a smaller arena.
http://youtu.be/JocFZ3nDSsM
5. Fantastic seamless set piece, exploration and gunplay transition.
http://youtu.be/I6R7dtiwneI
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Finally, I strongly disagree about Uncharted not being replayable. For me and countless others it has been one of the most replayable games out there.