Starting at high level has it's own issues.There's no mystery here. Mathematically speaking, it's inevitable.
You can always start at high levels, of course. But most campaigns don't. So getting there takes significant time and commitment, which means a lot of different opportunities for attrition. The game is designed such that, if you don't skip ahead, few campaigns will reach high levels just because of natural attrition.
My online group kept asking to try high level (when I joined group was 5-6th and hadn't ever done higher then that), DM didn't want to do it, so I ran a multi session 15+ level adventure.
I offered either they could make their own character or I'll make pre-gens. They all chose pre-gens. I told them to FULLY familiarize themselves with their characters, especially the casters. They had two weeks before the first session.
At the first session, it became immediately apparent that 4/5 had no idea what their characters could do. It was particularly tough for me because not only did I have to know what was going on on my end (more challenging because high level stuff), I had to constantly field questions about their characters. Made it through and they seemed to (eventually) have a blast, but it was pretty rough on my end (still not as rough as running high level 3e/3.5 though).
Point being, even though 5e isn't THAT crunchy, jumping directly into high level with no experience is tough for most.
It's why we're so impressed by campaigns that last for decades.
It doesn't have to be that way. World of Warcraft makes levelling relatively fast and easy, and then puts most of the effort into the end game experience. That approach seems to work out better for a video game than for a TTRPG.
Yeah, I'm running the group through the Planescape adventure, which has very fast leveling. They're enjoying it, but complaining that it's actually TOO fast. It also has a level jump (to 17th) and this time I'm making SURE they're prepared, honestly considering doing a pre-session Q&A/tutorial!