Why Op-Amp Saturation voltage drop with increasing frequency?
Your left operation amplifier with its RC components are an active low-pass filter - that's their purpose, if you think about it!

So what it mathematically  is is a "integrator". And that's the whole reason the amplitude of the triangles decrease with shorter period time (=higher frequency). When you integrate a constant (high input) for a long time, your integral will have a higher value than when you integrate the same constant for a short time.

Atop of that, real-world amplifiers also have finite bandwidths, and the gain of the semiconductor circuitry inside an opamp goes down with frequency, and at some point the idea that an opamp has "very very high gain" just breaks down. However, at your cute 45 kHz, that's probably not the situation here, unless you're using something extrodinarily slow (and then, you'd probably know – that opamp would've been labeled as "only for audio applications" or "vintage" or so).