To add to this—most of the marriages I've seen that have come after multiple serious relationships struggle with baggage from those previous relationships.
Often it's obvious things like kids, but it's also more pernicious things like expectations, comparisons, and even just different worldviews. A couple that grows up together can end up substantially more unified than is possible when you're joining lives after a decade of adulthood shaped by multiple partners.
There are obviously exceptions on both sides—first-timers that were toxic and 'experienced' partners who work well—but I certainly haven't seen an unqualified series of successes in the pattern described by OP.