Because they still get to collect the miner fees...
At this moment, the fees are not that much... For the two latest blocks:
https://blockchain.info/block/000000000000000002394538d6f5a2cf3fbf730c65ab1aecdc3f46a28629689f
https://blockchain.info/block/00000000000000000459e60ff756e66b1cc45a3769366121883a954d2a53d0e7
0,16 and 0.21 BTC
Offcourse, it is hoped that by then those 0.16BTC will have sufficient value to make some profit for the miners... If not, some miners will stop mining, the diff will drop, the rest of the miners that didn't stop mining will find more blocks, and collect more fees as a result
It's an equilibrium