I can only guess about the reason. Here's my guess, the most powerful spells should be rituals. Whether we like them to or not, any set of rules will have ramifications on the setting (even so called universal games). The Ars rules as they had been implemented in previous editions had a large discrepancy at the high end of things between what was practical and what was thematically appropriate. Rituals were only by story guide fiat (there were a few restriction in fourth edition). Magi effortlessly pulling off level 70 effects wasn't cool . Your example of DEO is not a particular problem but without the level 50 restriction a spell that turns people into frogs range sight duration moon target group (+12 size modifier, so as to affect every human in the world) is not a ritual if the SG does not want it to be.
I believe the language of rules says "generally" and leaves some wiggle room on the level 50 restriction. It is a guideline not a hard and fast rule.
Also "Any magus that can cast such a spell can probably create the item in a season, the same amount of time it would take to make the spell" is simply incorrect. (but It's true that it isn't a great deal more difficult to create a spell than to enchant an item).
Rituals have an excellent roleplaying element in that they feel mythic.
Having a spell that kills a man at arcane conection really isn't terribly mythic, but a ritual to do the same, with bubbling cauldrons, chalk hexagrams and such is.
It also makes the more powerful magic mean something as it takes resources (vis) to do. Inflicting a light wound on every member of Saladins army shouldn't be something you can just throw out, if you do it, it should be hard, it should take effort and it should take resources so you don't do it many times.
I'm neither one of the designers, playtesters or even secret masters of the game, but here's how I see it:
There's a tension in Ars Magica between the mythic and the gritty. On the one hand, we have magi capable of world-shaking magics. On the other, we have a large audience of players who want the game rooted in real history. That's gritty! Without caps on magical power, real world armies, castles and social power structures make no sense at all, at least not once the PCs arrive on the scene to overturn the medieval world and usher in a Renaissance fueled by Hermetic Magic.
Without a cap on magical rituals, what other than an irate GM's fiat prevents players from making sure the weather is always good... in Europe? Or casually defeating any mundane army?
That kind of thing might make sense in high fantasy, but AM strives for historical fantasy--with wizards.
Another solution might be to drastically reduce the power of wizards, say, so that lvl 30 spells become worthy of an archmagus rather than a minty fresh magus just out of apprenticeship. But that makes the wizards less... wizardly.
Forcing powerful magics to be rituals that consume precious resources addresses the problem nicely.
Why 50? Ten magnitudes seems about right. Maybe 9 or 11 would be better, but ten, you know, is ten.
New Hermetic Virtue: Eleven fingers--your magus has six fingers on one hand. Some people consider you cursed or unlucky, but you can cast spells up to level 55 without needing a ritual.
The idea of rituals has a lot of good tone for the game, and they needed a rule to ensure rituals had a good place in the system(--there is a direct relationship between system and tone or mood).
They set a mark that had some feel to it (tenth magnitude). It also ensured the most powerful effects would be reserved for rituals, suggesting the Order's basis in the Cult of Mercury. They didn't want everything to be a ritual, though, and certainly didn't want the SG to have to arbitrate every spell.