Frequency dependent behavior of an inverting configuration of an opamp
Op-amps behave like low-pass filters by design. Here's why:

In a typical small-signal analysis, we consider an input sinusoid of a given frequency, but in reality the op amp doesn't know anything about input signal frequency. Output reacts to input with a little bit of lag due to parasitic output resistance and capacitance, so the output signal has some non-zero phase shift as a result of the practical implementation details.

(Sinusoudal input is used for analysis because sine is well understood, and Fourier analysis lets us restate any arbitrary signal into a bunch of sine waves. Sines are the easiest way to deal with analysis in both time domain and frequency domain.)

Something important happens when phase shift reaches 180 degrees: the polarity changes, and convergent, negative feedback suddenly becomes divergent, positive feedback. This is bad, because it can unexpectedly turn an amplifier into an oscillator (or conversely it can prevent an oscillator from starting up).

What's worse, these kinds of problems are frustratingly difficult to diagnose and fix, because the problem is sensitive to any small changes in parasitic capacitance. (Capacitance happens between any two points that are not joined by a conductor, and parasitic capacitance refers to these small, ad hoc capacitors that appear literally everywhere.) Even connecting an oscilloscope to try to measure the oscillation, will affect the system. You can end up with a system that only works when you're actively trying to fix it. Bringing a finger near the system adds around 1pF of capacitance, which can be enough to influence a marginal design.

To steer clear of these problems, engineers use a design heuristic called phase margin to ensure that the closed-loop gain of the system falls below unity gain well before 180 degrees of phase margin are reached. Typical designs aim for 60 degrees of phase margin, that is a maximum of 120 degrees of phase shift. (A mathematician would also point out the Nyquist criterion for stability, but phase margin is a much simpler rule that is useful in most practical cases.)

Many op amps are designed with internal compensation capacitors to force the open-loop gain to decrease at higher frequencies, in order to make the system stable when used in unity gain (voltage follower) configuration. This makes the op amp much easier to use, though it does waste some bandwidth -- the uncompensated version of the same op amp can give higher gain, but will have a specified 'minimum closed loop gain' to be usable.