Colliding particles and activation energy If two particles collide and they are under activation energy, why do they JUST “bounce apart”? Isn’t the activation energy steadily decreasing if those particles continuously collide?

I always viewed the activation energy as a "barrier" that prevents collisions from producing a reaction, if sufficient energy is not met. I thought that if particles collide and they do not have enough energy to cause a reaction, the "barrier" gets weakened. Therefore, I found it surprising that this "barrier" doesn't get weakened, and the activation energy isn't reduced.
This is the nature of the quantum world. An action, at some level, either takes place or does not... there is no half-way state. Another example is the photoelectric effect. Planck and Einstein explained the requirement for at least a minimum energy of a photon before it can raise an electron to a higher energy level. A million photons just under that energy will not* raise an electron to a higher level, but just a single photon of the required energy (or higher) can bump up the electron.

Caveat: There is also the uncertainty principle, allowing electrons to tunnel to a higher level, or for multi-photon absorption to cause ionization, but they're not as  common as single-photon absorption and ionization.

If you're bothered by this non-intuitive behavior, you're not alone. Bohr, Bose, Boltzmann, Einstein, and others were forced to this theory through experimental results and the horrors of the ultraviolet catastrophe. Those who fail to appreciate the ultraviolet catastrophe shall be made, energetically, to walk the Planck.