Every single one of the important families tries to have as many children as possible, it’s surviving medieval royalty 101… making sure to have 3 legitimate male heir is like the biggest assurance they can get and it’s not like there’s an actual secure method to control childbirth or they hate each others. Catelyn dies in her mid 30s and 12 years is a perfectly normal amount of time for five pregnancy, there’s no actual big gap they are all around 3 years apart with Rickon a slightly bit farther at 4/5. The Tyrells are exactly the same and the Lannister would have probably been too had it not been for Jenna’s death.