The comprehension question is the easier one.
General relativity is an extremely mind stretching way to think about the universe. And, why shouldn't it be? Humans evolved in a particular niche of space where the equation g=9.8 m/s^2 in the down direction. Human had no need to have any more sophisticated understanding. Additional understanding of gravity imparted no evolutionary advantage.
Likewise, in all but the last century of the hundreds of thousands of years of history, human have had no occasion to encounter matter engage in anything other than chemical reactions, except for a few naturally radioactive sites that could be chalked up to the much more plausible explanation of evil spirits. Why should humans understand quantum theory? And, in particular, why should they understand either nuclear strong force or nuclear weak force?
Humans are not finely tuned perceptual creatures. For example, we can't tell the difference between vision at twenty-four frames a second and a continuous view of an object. As long as we appeared in a common part of the 4th dimension every 24th of a second, who would notice the difference? Even artists, for example, took well over a thousand years before they could draw horses in motion with their legs the way they really worked, for essentially that very reason.
Now, don't get me wrong. A 4-D universe, three dimensions in space and time, would be a highly convenient physical relaity. BUt, imperfect human peception isn't necessarily the best argument for its existence.