Multiple efforts can increase competition or spread resources thin.
We already have 3 major engines. Do we need a fourth? The Linux Kernel combines all efforts in a single thriving project, while we have multiple desktop distributions struggling to get a single digit percentage of the market.
One is closed source, so you can't even consider it for adoption. Google is now proclaiming its dissatisfaction with webkit, and Gecko is a 20 year old codebase now, and Google likes control.
Though considering how they fund Mozilla in the first place, if they wanted Gecko, they would just buy Mozilla. They keep the lights on anyway.
Two of the biggest players (and now arch rivals) sharing what's arguably the most strategic piece of code there is, couldn't last very long.
It's a shame though. It was probably the biggest open source success story.
An open source monoculture is not the same as a proprietary monopoly.