From my experience each industrial cycle (5 days by default) each shipping line checks to see if it has enough money to build a ship and then randomly decides if it will, then decides what to build by a percentage weighting between the 3 options (freighter, colony ship, luxury liner).
In my current game, one of the shipping lines is big enough, with enough ships in action, that it is making enough money every 5 day increment to pay for a new ship. In order for the shipping lines total funds to have a net decrease, the shipping line would need to be able to build more than 1 ship in a 5 day increment. The smaller of the 2 shipping lines in my game can currently sustain a new ship every 3-4 weeks and is doing that.
I am ok with this, I just wish I could ignore those message interrupts without also skipping the new Civilian Mining Center messages which I actually care about.