The biggest thing is that you don't need to just worry about other satellites, and the second is that the orbital material can exist at any altitude that satellites are orbiting at.

In terms of debris that can cause Kessler syndrome, 100k is a miniscule number. At the speed of a satellite, it does not take much to do serious damage and destruction. Your planes only have to worry about 100k planes, while satellites have to worry about motes of dust. If planes had to worry about that as well, we'd have serious issues flying at 500mph.

And United States Space Surveillance Network has identified this:

36500 space debris objects greater than 10 cm

1000000 space debris objects from greater than 1 cm to 10 cm

130 million space debris objects from greater than 1 mm to 1 cm

A single satellite being destroyed can be enough to turn 1 satellite into thousands, perhaps even tens or hundreds of thousands of pieces of individual debris that can then annihilate the rest at the same orbital level over time.

The change in momentum can be enough to cause concern.

This is such a very deeply important topic that it really does require a lot of protection, planning, redundancies, etc. SpaceX just doesn't seem like it has done enough to address this. There's a reason that NASA has moved more slowly and beat private companies to Mars, and it's because every single detail - every single detail of material science, chemistry, thermodynamics, physics, etc, has been meticulously vetted to the letter.

Falsely believing as a private company that "Wow, they're all foolish, this was easy, all we had to do was shove some satellites at this other altitude" and not putting enough merit in Kessler syndrome is a very big mistake.