Altruism. But reality is, without incentives, Bitcoiner's can't be expected to take/accept the opportunity cost to lock their capital, Bitcoins, in Lightning channels.
I don't really see big opportunity costs for Bitcoin hodlers if they provide liquidity for Lightning, because you don't "lock" the capital really, but more "freeze" it. Because you can always close the channel, and you are also not forced to be online 24/7 (although it would be ideal, obviously, if you did). So it's not really different if you hold the BTC (improductively) in your wallet.
There is more a direct cost involved, and that is the one of transaction fees (when opening and closing a channel). But you can do the opening transaction in a period with low fees for 1 sat/byte (currently always possible on weekends and most of the days also at night in the Western hemisphere). And even if you use your BTC regularly, you should be able to know which part of your balance you only need long term.
And I agree with Carlton Banks: long-term there is a reward, it's not only altruism.