Because like most machine exercises, it doesn't force your body (I.e. spine and pelvis in this case) into the correct position (neutral), only works your rectus abdominal and probably rectus femoris, and doesn't make your body work together as a combined unit (like a proper plank).
THAT is why such machines are frowned upon by weight and power lifters, as well as a good few bodybuilders.
You can't bench properly (I.e. with a barbell) because you have been using smith machines. These are popular because they are 'safe' and reduce the potential for somebody to injure themselves by not controlling the weight properly. Using a free weight forces many more muscles into action to stabilise the weight properly.
Take a leg press: some people on here can load them with hundreds of kg and rep them until the cows come home, but can't squat for toffee. Sure, they will have big thighs, but aren't functionally strong much in a real world scenario.
Does any of this matter to you? It should, but going by your attitude above (I.e. ask a question and then get very defensive when people criticise your choice of exercise - sure, a lot of it was very blunt, but you are hurting yourself performing an exercise not renowned for being sensible, suggesting you didn't do any reading before embarking on this venture, further suggesting you don't really know what you're doing) I suspect it won't.
It is good that you are training - great, even - but there are a lot of people on this forum who would make a lot of money training other people if they chose, simply because they know more than most of the PTs in this country put together. And a good few of them have posted in your thread.
As one of the well-known and (sort of) respected posters on here noted, the guys with the biggest physiques and best lifts on here all do a very similar set of exercises.
Are you talking about a bent-over row?