I have never met an experienced cyclist who was not extremely particular about how they perch their fundament upon their ride. And once people find a saddle that works they tend not to experiment very much (and arguably they could experiment more). And yes, different geometry and riding positions imply different saddles. But if you are on your third gravel bike or fifth mountain bike or ninth road bike you probably know what saddle will work, and probably have one in your garage.
You might also be surprised to know that there are
drop bar mountain bikes and
flat bar gravel bikes. And some companies will sell you the bike with the choice of handlebars and they adjust the geometry with spacers on the headset and during bike fit (because of course the reach will necessarily be different). Also, even within drop bars and flat bars there are an incredible number of varieties of shapes and sizes, and those small tweaks can make the difference between a bike that becomes an instrument of torture on a 100km ride and a bike that is still sweet to ride during a brevet.
A "no s#!t" story: during my recent bike fit for my new bike, at the start I commented that I thought the handlebars needed to be about an inch wider. After getting on the trainer and pedaling for about a minute, he said, "more like two inches wider". So I'm looking into that right now.
There is also
this crazy product which lets you install "grips" on your drop bars. I'm probably going to get some when I upgrade my handlebars.
It is interesting hearing debates about drop bars versus flat bars. Drop bar fanboys argue that you have more hand positions, which limits fatigue during long rides. The flat bar crowd argues that while drop bars have more hand positions, the flat bar position is more comfortable and natural. Both of them have a point.
As for fenders, if you live and ride in the sunny, dry southwest you probably don't need or want fenders. If you live in the soggy, muddy southeast the stock fenders on nearly every bike are a pathetic joke and will gum up and foul up in a very few miles. Or if you ride on the rainy side of the Cascades in the Pacific Northwest on all-weather glacier debris those basic fenders won't provide sufficient coverage. So for a lot of riders in a lot of places they are either wearing the terrain they are riding through or purchasing
halfway decent fenders.
On racks, until you've used a decent double-decker rack like a
Tubus Cosmo you don't know what you are missing. Because you can mount the panniers low or high you have a lot more control over the center of gravity and the stability and rideability of your bike while carrying a load. And when you mount the panniers low you can easily strap (or unstrap) stuff from the top of the rack without struggling with the panniers. Honestly very hard to go back to the generic no-name racks most bikes come with after you've used one. And that Tubus rack will easily outlast your bike. I can easily make similar criticisms about most stock front racks, with the added observation that a lot of them are unsafe even if very lightly loaded. And I've lost count of how many beefy-looking stock front racks on bikes have a chintzy 3kg weight limit.