Yeah blaming the customer instead of better accommodating them is not the answer. If I were running a dental practice I’d open at 5am, close for a few hrs mid day and reopen at 5-7 and be open Sat.

It's tough to imagine finding qualified employees who will tolerate a split shift like that, or who want to only work part time and take one shift or the other. So the dental practice would, almost by definition, be made up largely employees who couldn't secure employment in a business with traditional working hours - maybe even making it the least skilled dental practice in the area. In order to attract workers who can work at 9-5 dental practices, you'd have to pay more than those businesses, given the awkward opening hours. And then you'd hope you got enough patients out of the shift in hours to make some extra money. But a little extra money wouldn't be enough to go through all the trouble... there's no point opening weird hours and then doing "about as well as other dentists" because the weird hours introduce various overhead. So you have to somehow figure out how much more profitable the weird-hours business needs to be in order to deliver a profit that's significantly more than a traditional dentist. And if that profit difference is worth the extra hassle (in its various forms) to you as the owner of the business, sure go ahead.

Of course he/she can. How often do you see the dentist? Twice a year max. Instead of asking their boss: can I come in late tomorrow or work from home, they expect an industry to shift their hours. Not just all the dentists but their assistants and admin staff.

However, he is not alone being stuck at work 9 to 5, and the dental practice wouldn’t just accommodate him by shifting their hours but the majority of the workforce and thus win business and make everyone happier.

For sure, but you're asking someone else to shift their hours, are you happy to shift yours.

Whilst there's an economic element there's a large social/family element to the consideration which is why, IMO, you don't see many services time-shifting: particularly dentists where they are relatively well paid and usually able to direct their own hours.

Example I work in an SME that could make much more money working Sundays, but we choose to close and be with Church and family. Same sort of consideration; valuing social aspects far beyond their monetary value.

Not everyone will be happier with business hours expanding, especially low-wage workers I expect as the expanding hours will lead to lower QoL.