Because of variance.
If you expect a long term return of 3% you bet 3 units, 2% 2 units ect. If you bet 6% of your bank roll on a an expected 3% you're inviting variance to wipe you out in the long run because you are betting too much on one play. If you don't know what your expected return is on each play, then you have no business varying your bet size, and are asking for trouble if you do. Things even get more tricky when you're betting big dogs or big favorites because your expected return will come in with a ton of variance (dogs) or with less variance (big favs).
I'm not good enough to figure out what my exact edge on every play so I flat bet half of my average edge. I also stay away from big dogs and big favs, because you have to vary you're bet size.
Imagine having a 5% edge on a +1000 ML's and that's what you like to bet. You're not going to risk 5 units because you will put yourself in vulnerable position of going broke because of variance before it pays off, so the correct risk would be to risk 0.5 units to win 5 units.
Only about 4% of cappers are +ev cappers and probably less than 1% of that 4% can correctly vary their bet size.