Because it's subjective. Each has it's own strong points or weak points, depending on how you look at it. I would need to see where someone did a 'heads-up comparison' to believe that a Firehawk outperformed a Vette. Got any links? But obviously the Vette is the better car because it costs more. j/k
Seriously though... the cars are similar in some respects, but are very, very different overall. If you want to compare the 'top dogs' for performance, then you would probably want to compare a Firehawk to a Z06 or a ZR-1... but that's really not your point so I'll keep it to the 'regular' C5 Coupe and/or hardtop.
The LS1 engine that is found in the Firebird Trans-Am is nearly identical to the one found in the C5 Corvette. The difference was in the camshaft (slightly different) that allowed the Vette to run without an EGR valve. EGR = better emissions = less power. Why? Because anytime you are recirculating exhaust gasses you are displacing oxygen, which means you cannot make as much power overall for a given displacement or power cycle. (Thermodynamics and physics rule out every time.)
Pontiac also had a restrictive air intake on the Trans-Am, which caused it to make less horsepower than the Vette. Open up the air box on the Trans-Am, and it will make a lot more power. (SLP and others make great aftermarket air boxes... and SLP 'made' the Firehawk if I am not mistaken.) Those two things, along with a more restrictive exhaust, gave the Trans-Am lower HP numbers than the Vette overall. Make a few minor changes, and the Trans-Am really begins to wake up.
The other differences between the two cars are more dramatic, however.
As others have pointed out, the weight differences are noticeable between the two cars.
Trans-Am: 3517 lbs
Firehawk: 3470 lbs
C5 Coupe: 3246 lbs
C5 Hardtop: 3173 lbs
Z06: 3135 lbs
As shown, they aren't extremely different... but you will feel that extra 200-350 lbs if you track the car. A good rule of thumb is: 100 lbs = a tenth at the drag strip.
But more important than that (for racing) is the weight distribution. The Trans-Am has the engine and transmission up front, whereas the Corvette has the transmission out back, mated to an aluminum Getrag differential. The weight distribution front/back looks like this:
Trans-Am: 56/44
Corvette: 50/50
If you are into racing (autocross and track style events) then a 50/50 distribution is very, very hard to beat. A car that is heavy in the front will 'push' into turns, This is called 'understeer' and causes the car to go wider than expected through a turn. Heavy in the back causes 'oversteer' which causes the back end to slide out from you. Not good for track racing, I'm afraid.
Another difference is in the type of rear axle found in each car. The Trans-Am has a solid rear axle that moves as one unit. A change on the right side of the car will effect the angle of the left side. The Corvette has an independent rear axle assembly. This allows the car to handle changes in the pavement while maintaining an overall better balance, better grip, and an increase in the car's handling. Unfortunately, the IRS in a C5 cannot handle the shock loads associated with drag racing like a solid axle can... so depending on what you are doing with the car will determine if a solid axle helps/hurts you.
Where it gets crazy is when you look into the electronics. The Vette came with Active Handling, with uses sensors to measure yaw, lateral acceleration and steering wheel position. When it notices that the car is getting out of control, it uses ABS and traction control to correct oversteer or understeer.
The Vette also had a selectable suspension, which could allow you to change the firmness of the suspension with the click of a knob. (Tour/Sport/Performance) Touring is soft, Performance is nice and stiff.
The Vette also came with a heads-up display if so desired. It would display many of the items that are on your dash up on the windshield so you don't have to take your eyes off the road.
The Vette also has a Display Information Center (DIC) that you can use to access ALL of the codes in your system. No code reader is required, just hit a few buttons and display all diagnostic information on the dash. All of these features are not available options on the Trans-Am.
But ultimately... while both cars have some similar parts, the Corvette is in an entirely different class. I won't bash the Trans-Am, and I think it's a really good car and can kick all sorts of butt at the drag strip and in town... but once you cross over into autocross or professional racing, they aren't even close.
But don't take my word for it... search for "C5 vs. Trans-Am" and see what's out there. Better yet, drive both of them and see if you can tell the difference. I know I can... which is why I own one.
Mark