It's about power. If they DON'T open source now, with very little effort they have the option to in the future. They don't make money now, but maybe circumstances change? No one knows the future, and we don't know their plans.

But if they do open source it, then it will take a lot of hard work to revert it.

In other words, it's a company and this is their asset.

WhatsApp users who sync their contacts provide Meta with a social graph annotated with phone numbers.

If you don't think Meta makes money from the WhatsApp user contact network and linking that data to their other data sets, you are most certainly mistaken. There's tons of money in it, even now, before they start adding new privacy compromising features.