If you're talking about FAANG, which I assume you are, that's possibly true. But they aren't the only "big companies". There are tons of big cos which do not do any R&D (Cisco, Oracle and the like) and instead acquire startups to grow. And here's the kicker, they don't care for employee development; so choose wisely.
In general, because most technical startups are a wreck and otherwise have technical impedance mismatch with the acquirer, acquisitions likely have integration + technical expansion periods for the first few years before thy succeed/die.
Agreed with others: Opportunities are there, more about whether individuals go for them. And if you mean it doesn't just magically happen, yep. AFAICT, even at Google/FB/etc., PMs have to fight for who they get, and they probably fight for a lot of the same people, and the rest no one really knows...
Yes they are external entities, not part of Cisco. Cisco I believe calls them "spin-ins". They aren't Cisco employees though.