It's not a practical limit, it's a theoretical one. The original calculation that the 100km/62mile definition allegedly stems from was done by Theodore von Kármán, who determined that at ~84km/52miles, an airplane would have to move so fast to produce sufficient lift that it's speed would place it in orbit. The number was rounded up in most countries to 100km, though down to 50 miles in the US.

The fact that noone has actually flown a plane in sustained flight anywhere near that high doesn't change the math. Though I'd note that there have been unpowered flights at such altitudes - the Space Shuttle, Buran, X-37B, various hypersonic glide vehicles, etc.

Perhaps the best example is the Apollo capsule - not typically something you'd think of as an aircraft, but it did produce lift, and if you look at it's reentry profile you can see that it managed to maintain (approximately) level flight at around 200,000ft for a fair distance, before finally bleeding off enough speed to continue descending.

If the Apollo capsule, or Space Shuttle, or whatever had been fitted with some form of propulsion to maintain speed, then they could theoretically have sustained level flight at over 200,000ft - at least until their heatshields gave out anyway.