Okay, here are a few thoughts:

• Companies like blame someone when things go wrong, if they chose open-source there's isn't someone to sue then;
• Buying proprietary stuff means you're outsourcing the risks of such product;
• Corruption pushes for proprietary: they might be buying software that is made by someone that is close to the CTO, CEO or other decision marker in the company, an old friend, family or straight under the table corruption;
• Most non-tech companies use services from consulting companies in order to get their software developed / running. Consulting companies often fall under the last point that besides that they have have large incentives from companies like Microsoft to push their proprietary services. For eg. Microsoft will easily provide all of a consulting companies employees with free Azure services, Office and other discounts if they enter in an exclusivity agreement to sell their tech stack. To make things worse consulting companies live of cheap developers (like interns) and Microsoft and their platform makes things easier for anyone to code and deploy;
• Microsoft provider a cohesive ecosystem of products that integrate really well with each other and usually don't require much effort to get things going - open-source however, usually requires custom development and a ton of work to work out the "sharp angles" between multiple solutions that aren't related and might not be easily compatible with each other;
• Open-source requires a level of expertise that more than half of the developers and IT professionals simply don't have. This aspect reinforces the last point even more. Senior open-source experts are more expensive than simply buying proprietary solutions;
• If we consider the price of a senior open-source expert + software costs (usually free) the cost of open-source is considerable lower than the cost of cheap developers + proprietary solutions, however consider we are talking about companies. Companies will always prefer to hire more less expensive and less proficient people because that means they're easier to replace and you'll pay less taxes;
• Companies will prefer to hire services from other companies instead of employees thus making proprietary vendors more compelling. This happens because from an accounting / investors perspective employees are bad and subscriptions are cool (less taxes, no responsibilities etc);
• The companies who build proprietary solutions work really hard to get vendors to sell their software, they provide commissions, support and the promises that if anything goes wrong they'll be there. This increases the number of proprietary-only vendors which reinforces everything above. If you're starting to sell software or networking services there's little incentive for you to go pure "open-source". With less companies, less visibility, less professionals (and more expensive), less margins and less positive market image, less customers and lesser profits.

Unfortunately things are really poised and rigged against open-source solutions and anyone who tries to push for them. The "experts" who work in consulting companies are part of this as they usually don't even know how to do things without the property solutions. Let me give you an example, once I had to work with E&Y, one of those big consulting companies, and I realized some awkward things while having conversations with both low level employees and partners / middle management, they weren't aware that there are alternatives most of the time. A manager of a digital transformation and cloud solutions team that started his career E&Y, wasn't aware that there was open-source alternatives to Google Workplace and Microsoft 365 for e-mail. I probed a TON around that and the guy, a software engineer with an university degree, didn't even know that was Postfix was and the history of email.