Because it's -EV .
I once did the math for TT, he needs to 3bet 10%-11% at least to make 4bet/call profitable. If you want I can do the math once more, but imo you should also try it for yourself ;).
Ok, I think you meant 4bet/fold when you said it's -EV (that's what I was asking about), but maybe both 4bet/fold & 4bet/call are -EV so I'll try to see which is worse.
Assume eff stack of $100, we raise $3.00, villain 3bet $10, we 4bet to $24...
Assume villain 5bet jam 3.02% {JJ+, AKs, AKo}
Therefore villain folds 2/3 of the time to our 4bet and jams 1/3 of time.
Cost of 4bet = $21 ($24-$3). So...
4bet/call (same as a 4bet shove where villain calls):
0.666 * ($3+$10+$1.50 = $14.50) = $9.67 plus
0.333 * (0.332 * ($100+$3+$1.5) = $34.694 less (0.668 * $97 = $64.796) = -$10.02
total ev = -$0.35
4bet/fold:
0.666 * ($3+$10+$1.50 = $14.50) = $9.67 less
0.333 * $21 = $6.993
total ev = $2.68
So it looks like if villain's 3betting range is exactly 9% and his 5bet shove range is exactly 3%, we can profitabaly 4bet/fold, but should not 4bet call with 99 or TT. Is my maths correct and do you agree with the conclusion?
p.s. plse check my figures - there is normally a mistake 
pp. I just checked this on Xarry's spreadsheet and I think it says I need 69.25% FEQ for b/e which suggests I have got the maths wrong
since we only get 66%