R&D usually is budget constrained at the company or division level (% of revenue) and you can only ask for it once a year. Next year's budget time determines if you get more or less. Time constraints come indirectly (proof of progress for budget expansion or more importantly declining revenue from existing products),

But the only way management knows how to hold R&D accountable to ship is with dates as a forcing function, and those dates are often invented or organized around industry events (conferences, press events, etc).

There are other ways to manage progress, dates are the most common lever. That can work but can be abused by bad management. I've usually preferred shops that say "it ships when it's ready", but they require special circumstances to maintain funding and measure progress. In general if what you build is more important than when it ships, "it ships when it's ready" is better than hitting a date with a dud. So long as there's value for the budget and a way to measure it.