On the one hand it does seem a bit weird that one character could lose 25 feet worth of movement standing up while someone else might only lose 10 (if they had a speed of 20'). But then again... if we take away the knowledge of how much a PC should be moving... two characters stand up from prone and one only gets to move 10 additional feet, while the other is moving 25! That's a huge difference.
And there's not really a way to "fix" it, because even if you were to change it from losing half your speed to losing a set amount of feet, you end up with the same problem but reversed... this time in terms of proportion. If you were to say that standing up from prone requires 10 feet of movement... the PC that moves 20 feet per round loses 50% of their movement just by standing up... whereas the person who moves 50 feet only loses 20%. And thus you then have to ask why does a faster walking person stand up so much quicker than one who doesn't?
On top of that... you then also have the issue that by making standing up only a certain number of feet, you are making it much less of a punishment for a faster character. A monk who moves 50 feet a round loses very little (proportionally) from having to stand up. Being knocked prone is no longer much of a punishment. The monk is still going to get to move 40 feet, which is still faster than almost every other character out there. By taking away a full 50% of every PCs movement... being knocked prone is an equal hindrance (proportionally) to every character. Yes, the faster character still gets move movement left over after they stand up... but they aren't proportionally even faster than a slower PC.
So while it seems counter-intuitive that a monk should "lose" so much speed just from standing up... in truth their walking speed is just so fast that even this "punishment" can be overcome. Your wife still gets to move 25 feet after standing up... while her platemail-wearing party member is possibly only going to get to move 10'.