Why not? If the AI is perfect you would never win. If the reinforcement rates are lowered then you would have a whole new batch of complaints from a different set of people who will complain that they can't win as the King of Nubia just to name one where those fast reinforcement rates are vital for surviving. I am actually all for it as it will make the game harder not easier. Once I set up a kingdom with more than 20,000 in levies I know I will win. The original poster makes his case with probably a small kingdom against the HRE or a bigger kingdom. You can do what icon41gimp suggested and win easier or just smash the reinforcements again and again until they are all gone and it is not that much harder. My first couple of games as Sicily with lord mayors I actually outnumbered the HRE. I got bored and played straight up the only "gamey" tactic massing my troops to one flank with a good general. Declaring war on the HRE with levies of only 25,000, I lowered his levies from 180,000 to about 17,000 and won. The fast reinforcement rate is what helped me win since I did not win all the battles. Of course bad AI management of doomstacks helped but then again if knew the AI was good at that I wouldn't have declared in the first place. How? Those levies are determined a lot by opinion, as a small kingdom your levies are either totally yours or if you manage your kingdom correctly your vassals will love you. HRE Emperors who lose a lot of battles and constantly raises their levies don't have that. It is hard to please that many people for an AI Emperor. The game like all things will never be perfect for everyone.
A minor tirade which might help you in your game if you find this game difficult: you can set up your kingdom with only lord-mayors for example and buy your way to having a larger army than basically anyone, how is this working as intended? Assassinations can still be overused once you have enough money from using point 1. Just group your army to one flank and beat larger armies regularly. Is this historical? Generals only did that as a last resort gamble but it is basically the only strategy that will win regularly with a good martial skill in the game. I think that is the lesser evil in roleplay as at least I have to get a good general (which is easy enough anyway) to lead the armies that win regularly. At least I can make the case that the martial skill is what makes the difference. I make it a point never to assassinate except by plot and only if I have a high enough percentage. I never appoint lord-mayors once I realized how powerful they are.