I agree with most of your positions and I also mentioned location as one of (IMO) primarily reasons. The original question itself is too vague and depends on our own countries of residence and personal experiences. Access to nice dive sites from the shore also reduces the costs significantly, absolutely agree.I think a lot of this is location dependent. If you live in an area with good shore diving, then that lowers the costs significantly. Where I live shore diving is not really a thing. West coast of FL has a very gradual slope. From where I normally go, I need to run out at least 10 miles from shore to get to water that's in the 50' range. A bit north of me it's even worse. Rule of thumb there is closer to a foot a mile.
If I want to do a shore dive, I need to go to the other side of the state. Most likely need to factor in a hotel stay as well.
So, all that adds up.
Cost of certification.
Cost of gear and annual maintenance (or rental costs per dive).
Cost of dive site (charter or dive site fees), or in my case cost of boat, fuel, maintenance.
It's not cheap.
The motorcycle example is also location dependent. Just going by what is required here, diving is more expensive. Not saying that the minimum requirement is enough, but it is all that is required.
Motorcycle endorsement on your license (pretty cheap, I think it might have been under $50)
Helmet (or if you have adequate health insurance, this isn't required)
And of course a motorcycle. (My first motorcycle cost significantly less than my dive gear)
Regarding motorcycles been related to location I tend to disagree. Most bikers buy motorcycles because they have easy access to nice locations and / or their friends already ride bikes. The 'Lobo Solitario' attitude is pure BS. We predominantly ride with friends, the same as with diving.
Regarding the cost of ownership, again, huge topic. If you are talking about a Vespa or little moped, of course things change BUT if you are talking about a 'real' motorcycle, things add up pretty fast. Tires and maintenance are also exuberant compared to servicing your regs every 1-2 years. A good set of sport rubber can cost as much a a good regulator and depending on how and where you ride, last only a few weeks...