The most successful companies have lots of cash, high share prices, and amazing cash cows. They could borrow for (almost) free, so resources are practically unlimited. Their R&D is already well funded. Most of their big, growth oriented endeavours are not cash-constrained. There are usually no factories to build or production to scale up.
Google tried "20% time." They tried "let many flowers grow." Those things seemed ambitious at 2007-scale. In 2021 terms... new flowers need to be S&P 500 companies to represent growth, instead of just clutter. "Meaningful growth," for Alphabet, is a big number.
How else does a MSFT, Google or (especially) FB put $20bn to work? Acquiring "just works."
Of course, there are in-house alternatives. Waymo is an in-house investment by Alphabet that's bigger than this Nuance acquisition... especially if you consider the $bns Waymo will continue to need until some unknown future date. Self driving is looking more hopeful (certainly to investors) than it was when waymo started.... but waymo is still a dubious investment.
Consider that Google could have bought any car company, for about as much as waymo will cost eventually. Car companies have loans, so you could quibble the math... but details.
Acquiring is easy. The path of least resistance wins >50% of the time. We have that dynamic here, both in the human/managers sense and in the arbitrage-like incentives in the market currently.