Liquidity should increase with the number of traders (or speculators) using the platform.
New exchanges usually offer either reduced fees or no fees at all for specific periods of time to get the traders come there.
Some others offer certain bonuses in coins that cannot be withdrawn, but can be used to pay the trading fees.

I guess that some liquidity providing bots can be also be made in a similar way as the arbitrage bots work, just their owner has to configure them very well in order to not lose too much money and also stay at good prices for the coin pairs.