Do you think that dentists don't also have a reputation to protect? What makes you take polar opposite views of the two?
I think people are also much more likely to talk to one another about the "shitty new Walmart healthcare service" because they know everyone knows Walmart and might use it. It's far less likely that you warn people about one single DDS office in your town unless perhaps that specific service provider came up in conversation.
Most dentists I've dealt with run their practice as a small business. Usually, it's a few dentists. They need to constantly sell. I understand that they need to stay in business but it's so constant that you never know where you stand.
I'm not saying that all dentists are like that it's just hard to find one that you can trust. Just like car mechanics.
What makes you think that Walmart is different here?
Walmart, on the other hand, actively has a target on its back. And with $524bn in revenue last year and almost $33bn in EBITDA[1], it's one of the most deep pocketed targets around. Any minor issue has the potential to spin itself into a national news story, and malpractice lawyers would be giddy to nab a patient they can use to take a stab at Walmart's pursestrings. And just think of the stars in their eyes if they stumble upon enough complaints to get certified as a class action and be able to really dig into the clinical guidelines and operating procedures for deficiencies to exploit.
Not to say the service they offer is likely to be world class. But between their existing institutional competencies in operations[2] and the potential liability concerns, it is likely to garner a reputation for being consistently and predictably acceptable[3].
[1] https://www.macrotrends.net/stocks/charts/WMT/walmart/revenu...
[2] Walmart's operational standards are notoriously demanding and exacting across their supply chain network. They could easily bring that to bear in running their clinics and enforcing compliance with clinical guidelines and SOPs from their medical providers.
[3] Walmart's reputation for mediocre product quality isn't an accident, it's a deliberate choice. And one that takes a considerable amount of effort to enforce and consistently maintain across the board for decades. They use their scale to get an incredibly detailed view of consumer behavior (because what people say rarely matches what people do). Then turn around and squeeze their suppliers to get a bill of material that minimizes return rates and maximizes consumer satisfaction within the constraints of price points determined by prior consumer behavior.