I seem to parry much better one handed
Jeanry, the question is, how do you parry? Do you have a copy of Medieval Swordsmanship: Illustrated Techniques and Methods by JC? Reading on the section about longswords, I'd suggest that when parrying properly, using two hands does have its advantages, particularly because of the stability it offers against the force of your opponent's strike. I'd suggest that a one handed parry against a proper two handed strike would indeed be at a disadvantage. Of course, I am still relatively new to HEMA, and I have not been able to attend an ARMA gathering. Perhaps a more experienced scholar can comment upon what I've said.
Also, you asked why sword makers wouldn’t simply add more length if two handed swords are inherently superior. However, it is crucial in this discussion that we also look at it from a historical perspective. For most of the time that is considered to be the Middle Ages, the armour in use is mail, not plate or a mixture of plate and mail. Mail does have its defensive merits, but used alone, it does not have tremendous value. It is still possible to suffer tremendous wounds from the blunt trauma of the impact of a sword. However, a shield coupled with mail suddenly allows the warrior to be far more effective in combat. I suspect there are some people who would rather fight with a shield alone; it can be used to cover many different lines of attack, it can smash your foe’s weapon aside to allow for a counter attack, and it can even be used as a weapon in its own right. Mail, on the other hand, is simply a passive defense system that reduces the severity of a wound you receive should you be hit. Of course, if you’re fighting with a shield, naturally a sword with one hand is much more useful than one which allows two hands, so there’s not too much reason for the latter to be used extensively.
However, armour was constantly evolving, and soon plates began to be added to mail. Armour made of articulated plates, even at the time, could not be effectively defeated with the knightly sword of earlier times. Instead, one needs to be able to exploit the various places in armour where flexibility is needed. Suddenly, a two handed sword begins to make more sense- the heavier armour means that a shield is less useful than it once was, and longer swords capable of being used with two hands allow greater offensive potential with cuts, but more importantly, using two hands offers much greater control when thrusting to exploit plate. And of course, using a sword with two hands does have numerous advantages, as mentioned by the others here, that a single handed sword does not.
I hope that helps to address part of your question.