I once read that color schemes were purposefully created to be either inviting and welcoming or to make people feel unconsciously uneasy and want to leave.
As for the modern architecture choices, I'm not really sure. But I recently visited a McDonalds. It very much looked like what you just said. These places once looked like they had family appealing character. But no longer. The walls looked like... all the character had been sucked out. Maybe more prison vibe than anything. It made the Happy Meal toy display look very out of place, like a relic to happier days. Never mind all the signs that softly yelled "Order and get out"... after 30 minutes. The place just felt like it was uninviting. And they had remodeled it to look that way, too.
Not to mention that the mere shake and a small fry cost nearly $10! I wasn't just going to use the restroom (which needed thumb print ID from a worker with a pin) and leave. But I don't think I'd ever go back... unless I had to. And McD's wonders why nobody is buying their food, any longer.
Does make me ponder if it's Covid cleanly capable, though. Sure felt like a choice that might be inspired by that curse.

Because at lease based on my observations, it's about efficiency and techniques to most accurately, swiftly, and efficiently to obtain objectives in a monetary sense from the stand point of these corporate bodies.
The bland paints and brutalist architecture, in themselves are more efficient than angled roofs, circular shapes, or Coloured paints, which the more pleasurable aesthetics costs more too.
In software and website design, they don't hire artists as artists anymore, they're expected make the UI and interface simple so that they can be scaled and adapted modularly for multiple devices types and resolutions, thus they must make aesthetics bland and simple for the most common denominator.
Spontaneous and non rigidity is inefficient because they are less predictable thus all aspect in business and even government planning must reduce all variable to the predictable and efficient, be if all for the stake of cost and time reduction. It's an evolution of "progress" in techniques.
It's natural psychological nature for one to grow tired of concepts that have been established and thus new technologies and new techniques pushing and forcing out the old when in terms of perception of friction and effort. Thus Corporations ban the use of typewriters when computers came around in the 80's and now AI automation is the one of the next new thresholds that must be met along with other techniques like blander more effective for profit margin architecture and interior designs.
that's why I feel they are going for all these bland and "modern" aesthetics. I wouldn't be surprise if it's also to reduce customers/consumers from spending too much time in their shops and restaurants when corporations want them to use their apps and automated services to get the consumer's services and products, to discourage people from sticking around in restaurants, chatting, laughing, and making a mess, wasting time when they could get the next customer to buy their "goods".
When there's more money to be have, It must be conducted. They will take that opportunity when the future states a robot on wheel can efficiently deliver the food and expects you to leave as soon as you're done eating the furtherly degraded in quality food in the parking lot, because of course the food must be more cost efficient too to maximize profits. Of course the quality of goods are going down because less material increases profit margins.
All as an excuse to enforce automation for efficiency's sake.

Many people miss the aero futiger and skeuomorphic days, but many don't realised that corporations only used that as a technique to attract people in when online subscriptions, streaming, and app based delivery methods wasn't developed enough at the time. If the current techniques were matured enough back in the 2000s they would of deployed them back in the 2000s.