Even if the windows were super thick, it wouldn't be strong enough to handle the pressure exerted by a properly positioned ladder.
A ladder is supposed to be sloped 25%, like this:
With someone standing near the top of the ladder, that means roughly 20% of his weight is directed as lateral force, directly into the wall or window, conveyed by the points of the corners. That could easily be hundreds of PSI, enough to crack almost any glass. That force is temporarily multiplied as each step is taken with something like acceleration and deceleration (this is the basics of physics kinematics, F = ma).
Wrapping the corners of the ladder with something soft (like a thick towel) would greatly reduce the pressure by spreading its force over a much larger contact area. However, the same total force is still applied, and that could well exceed the loading limit of the glass or its frame.
Leaning against glass is a high risk, low benefit undertaking. If the window breaks and the ladder moves, will that make you fall off? Will sharp, heavy pieces of glass fall onto you or be blasted into your face and eyes? Will you fall onto a window shard still held vertical in the frame?
There are many solutions which are easy, painless, and relatively inexpensive:
- Lean the ladder beside the window
- Get a longer ladder which can rest on the wall above the window
- With climbing gear, belay yourself from above
- Rent scaffolding, a cherry picker, or a self-supporting ladder like an A-frame or orchard ladder.