the reason is path lengths, sound travels relatively slow, & when the mids are up high, the left one is usually much closer to you that the right one..., which means that the sound from the left will reach your ear before the sound from the right, skewing any kind of sense of central location.

some will use time delay to delay the sound of the closer driver, this delay is only several milliseconds, but enough to put you back in a virtual central listening location. the biggest draw back to this approach though is, by delaying the left you can improve imaging to the driver, but at the same time you can make it even worse for the passenger.

in competition no one cares what it sounds like to the passenger, it's all about getting that sweet spot at the judged location, which is why a competition car will be more likely to use this approach (red & durwood, perfect examples)

by putting the drivers as far from you as practical though, you are evening out the differences of these lengths quite a bit, most who sware by kick installs are less likely to use delay for this equalizing of pathlengths, & that is why you'll see some really sware by kick installs, kick installs are a bit easier to get to sound good from both sides of the vehicle, although it will usually not be as great as if it was tuned for just the drivers seat alone, it's more of a comprimise & a matter of priorities.