For us, an accurate delivery date on a 6 month project was mandatory. CX needed it so they could start onboarding high priority customers. Marketing needed it so they could plan advertising collateral and make promises at conventions. Product needed it to understand what the Q3 roadmap should contain. Sales needed it to close deals. I was fortunate to work in a business where I respected the heads of these departments, which believe it or not, should be the norm.
The challenge wasn't estimation - it's quite doable to break a large project down into a series of sprints (basically a sprint / waterfall hybrid). Delays usually came from unexpected sources, like reacting to a must have interruption or critical bugs. Those you cannot estimate for, but you can collaborate on a solution. Trim features, push date, bring in extra help, or crunch. Whatever the decision, making sure to work with the other departments as colaborators was always beneficial.
Things rarely went to plan, but as soon as any blip occured, there'd be plans to trim scope, crunch more, or push the date with many months of notice.
Then I joined my first web SaaS startup and I think we didn't hit a single deadline in the entire time I worked there. Everyone thought that was fine and normal. Interestingly enough, I'm not convinced that's why we failed, but it was a huge culture shock.
I think you were estimating time to build things that were out of R&D and you had specifications that were actual specifications you were building up to.
In SaaS my experience is: someone makes up an idea not having any clue how existing software is working or is laid out, has no specifications beside vague not organized bunch of sentences. Software development team basically starts R&D to find out specifications and what is possible - but is expected to deliver final product.