One of the core conceits of D&D is: Level Matters. While playing D&D, players assume that Level plays a real, logical, and consistent part in determining the outcome of events. The higher your Level, the better you are at doing things. No player would expect a Level 1 Fighter to kill a Level 25 Fighter under normal circumstances, just as no player would expect that Level 1 Fighter to have the same ability to shake off a Sleep spell that the far more experienced and hardened Level 25 Fighter has. Instituting a flat 55% success rate on saving throws would undermine the difference between a Level 1 Fighter and a Level 25 Fighter when it comes to resisting Magic, thus violating the core principle that Level Matters.
A flat 55% success rate on saving throws completely ignores Level, thus violating the verisimilitude of the D&D game world. If a Level 1 Goblin peon with 6 Wisdom has the same chance to recover from a Sleep spell as Asmodeus, Lord of the Nine Hells, then the system you are using to adjudicate such a thing violates the spirit of D&D. In fact, I would go so far as to say that any system that yields such a result is not D&D, pure and simple. We can bicker about "Whatever Wizards Rubberstamps as D&D == D&D" all day long, but part of the core gameplay that makes D&D what it is does not allow for such a saving throw system.