As far as Marvel's situation, they haven't ever really done a "reboot".
That's completely inaccurate. At the beginning of Secret Wars last year, the Marvel Multiverse was destroyed. Dr. Doom cobbled together a kingdom, saving (or recreating) everyone he could. A handful of heroes who survived the destruction in lifeboats orchestrated a civil war against Doom and won. In the end, the new unified universe was created by Franklin Richards. It combined elements of the traditional Marvel universe, Ultimate universe, and the cinematic universe.
It was a full-scale reboot.
You are absolutely right, there was a lot of condensing of universes and definitely a fresh start again, I guess it comes down to what you consider a reboot. Comic book stuff is always murky in that way to me. I look at something like DC's New 52, where not only was every comic restarted with #1 issues but the previous history was essentially erased or rewritten as well. Batman had only been operating for 5 years, but still had all the Robins at some point. The Justice League hadn't gotten together yet, and when they did assemble Cyborg was a founding member and Martian Manhunter wasn't anywhere to be found. Wally West completely disappeared, then came back as essentially a different character...there wasn't a JSA, etc. there are lots of examples, but they essentially took the approach of "this is the first time any of this happened, or any of them met, etc.". They even went back and did a "Zero Year" storyline with Batman.
The Marvel stuff is a little bit different. Yes, they consolidated the Ultimate Universe with the 616 (and others), essentially plucking their favorite stuff or versions from throughout worlds or storylines (Miles Morales, Old Man Logan, etc., but the overall Marvel history was still in tact and generally the same. The characters still know each other, have extensive histories (as in they didn't just start being heroes and didn't just form teams), and the stuff that happened before still exists. The various Marvel comics still reference things that happened prior to Secret Wars, or even the Secret War itself in some cases. A fresh start for sure, but not necessarily a total reboot to me. Again, it's all in how you look at a reboot I guess. Like back when Marvel started up the now gone Ultimate Universe, if they would have quit publishing the regular 616 books and just stuck with the Ultimate U as the Marvel Universe, that is a hard reboot to me. The characters initially are based off of classic versions in most cases, but the stories became very different.