Although it would be realistic, in the current game it would also be rather pointless. You can only assault a UFO once, then it disappears from the geoscape, regardless of the outcome. So you turn a landed UFO into a crashed one, abort mission and then it's gone.. much like if you had just aborted the mission without the extra coding work of turning it into a crashed one.
If UFOs remained after assaults when the outcome is an XCom defeat, so that you can assault it again, that could be more useful. But getting multiple tries at an UFO changes the game at a strategic level. Then we would need the aliens getting reinforcements from another UFO (coming to rescue them) or the MiB (coverup) to balance the strategic changes and all that would require a lot of work...