Because he is still our best batsmen?
He has been consistent since coming into the side at opener, but his form has been tailing off. His last three series (Pak, Ind, Eng) have only produced 394 runs @ 33 with a high score of 83.
Very hard to argue he would be our best bat when he finally does come back having been out for many months and unlikely to have played any FC before he returns. And even so, how long do really reckon a 36 year old has left in his legs?
Waugh and Border made it to 38, but they were nothing like their best in their latter years.
After the 2001 Ashes Waugh was nothing like the run machine he had been with poor series against SA (twice) and England and fairly average series' against Pakistan (a 100* and 2 ducks) and India his last series. Only some cheap runs against the minnows held his average up at this time.
Border had a great last Ashes campaign at home and did well against the Kiwis but the South Africans sorted him right out and closed the curtain on a great career.
As good as Kat has been he is no Waugh or Border and the side can't afford to be hanging on to pensioners trying to pretend they're 30 and in there prime. Especially as we are likely to still have Hussey or Ponting or both still in the side.
Kat's injury is a blessing in disguise for the selectors in that they can take the opportunity to move on one of the old men of the side without having to make the hard call. I reckon they'll take it.