This is not completely true. There's one other reason to mine than the reward: because you recognize the risk of a hostile entity controlling too much mining power.

If you have 300 BTC and you suspect someone else is almost able to mount a 51% attack and thus devalue all your BTC, it makes logical sense for you to put more money into mining, even more than you can gain from the combination of block rewards and electricity, to secure your existing investment. As you do that, others will be forced to as well to avoid you having too much power and to protect their BTC and so on....