The same reason you use Chrome and then also Google Maps and Gmail. Just swap a Mac with Chrome and an iPhone with Gmail.
There are lots of little features like being able to copy on my iPhone and then it’s saved into a shared clipboard so that I can paste into my Mac. Those kinds of things (there are many, many others and you can look them up for yourself) are why someone would be influenced to choose an iPhone if they are using a Mac.
As an aside I think most users across all platforms use Google’s ecosystem but I would expect Apple users to use it less than others since Apple provides good enough alternatives such as Safari, Apple Maps, iCloud, etc.
Yeah this is what I was thinking. Apple users will tend to use two ecosystems instead of one.
Personally, I wouldn't even think about my laptop when choosing a phone. But I would think about the fact that I use several Google services and I'd rather not duplicate them with Mac accounts/apps.
Now I'm sure all of these things are possible on other platforms using extra software but in the ecosystem it's all zero setup, instantly integrated and quite reliable.
Honest question: what ecosystem are you referring to? Apple watch? Additional hardware you bought? Or are you talking about software?
Because my guess would have been that Apple users are typically embedded in Google's ecosystem as well (Chrome, Maps, Gmail, YouTube, etc).
And it's not clear to me how using a MacBook would influence your phone choice.