Because Google's production systems aren't internally open like that, other teams can't just go grepping through production data systems (which is encrypted at rest anyway). Yes, there are obviously systems which do index data for search, spam prevention, etc, but those are guarded too, and the surface area of access is greatly reduced.
You can't just decide "Hey, I've got an idea for a cool 20% project, I'm going to try crawling all mail and looking for stock market tips". There is defense in depth, in terms of technical walls, as well as people walls, that makes something like that very unlikely. With tens of thousands of employees, it would be easy to get 1 rogue employee who could do serious damage with that kind of access.
Google takes data security and privacy very very seriously, and a breach of that magnitude would threaten hundreds of billions in valuation for a rather crazy risky idea for an M&A department?
BTW, Google Ventures has a better way to do this which relies on public data, Project Sandhill.
By looking at data from Crunchbase, Angelist, etc you can get a deeper graph of the interrelationships in Silicon Valley. If I was looking at who to invest in, or M&A, I'd be looking at data like that, as well as talking with investors and people they've worked with.
"Which startups are good acquisition targets?" is very much a data-mining problem, though I would expect that noticing a startup experiencing "hockey stick" growth based on the growth of their Google Cloud Platform bill (or perhaps the # of users logging in with Google credentials, etc.) is much more likely than any Google employees (no matter how highly placed) trawling through Gmail looking for proprietary information.
