FormerlyHemlock
Hero
What I've found, and I know this is just me, is that the time spent gaming to go from level 10 to 11 is a few sessions, so it's an actual significant time sink of not playing the PC I really want just to get to a certain "build level". I.e., the "take a couple levels of X" argument. A couple levels is a couple months of gaming.
That's one factor. The other is that we rarely play above the low teens. 90% of our gaming is between level 3 and 10. So I'm sure that's a factor too.
I think the second factor is more important. The first factor is actually an argument in FAVOR of multiclassing for many characters. E.g. a ranger 11 who gains a level can either gain a feat, or gain IIRC Expertise in Stealth and Athletics, a free skill, and 1d6 sneak attack. A feat isn't terrible but at least to me, +4 to Stealth/Athletics and Sneak Attack is better. Next level Cunning Action is still more compelling than a feat, and then Assassination after that. Ranger and Rogue are both front-loaded so they multiclass well with each other, never leaving you stuck playing an "intermediate" build. Paladin and Sorcerer are similar IMO, with Paladin 1 through 6 (aura) then Sorc 1 (Shield, Expeditious Retreat, Fire Bolt, Tides of Chaos) being the logical first steps, then either Paladin 9 or Sorc 4 are equally attractive next goals, for either Aura of Vitality/Dispel Magic or Feat + Enhance Ability + Metamagic + Web/Blur.
YMMV though.