As the quora question here - https://www.quora.com/How-did-Bill-Gates-own-such-a-large-sh... - says, its for a pretty straightforward reason, economics.
If you need investor money to grow, you get diluted. If you don't you don't.
If you need investor money to grow, you get diluted. If you don't you don't.
They may lose money in the short-term but it allows them to become the de facto standard for telephony APIs and then they can increase their pricing or they can leverage their size to open new revenue streams.