As an occasional home roaster: the darker the roast the more the coffee loses the flavors related to the character of the original bean and the more they take on the flavor of the roasting process.
One of the major advantages of home roasting is that you can get premium beans from small farms (for relatively cheap as well!), this offers flavors and quality that are only available to small coffee shops simply because their aren't enough beans produced for a nationwide chain to offer to all their shops.
However, if you're getting high-quality, single origin beans it's much better to aim for a lighter roast since you will be tasting the (often even fruity) flavor of the original bean. This is also why small, high-end coffee shops tend to favor lighter roasts.
But even if you're a regional chain, you will likely struggle to provide consistent flavor from small batches so you'll, at the very least, work with larger farms. At the scale of Starbucks you're going to have trouble sourcing anything from a single farm, and you also want year over year consistency. Often small coffee shops pride themselves on short term offerings, because they're appealing to an audience that understands and values this. However the average Starbucks consumer not only wants a coffee in Alabama to taste the same as Seattle, but they want coffee in 2025 to taste like they remember from 2015.
The only way to achieve this level of consistency and such a scale is to create extremely dark roasts.
edit: there are also, of course, flavor related and cultural reasons to choose darker roasts. Italians, for example, tend to favor darker roasts for espresso (in part because they can create blends using varieties like robusta beans, that under a light roast have a 'rubbery' taste, but can add boldness when mixed in with a dark roasted espresso blend).
I think instead it has to do with the percentage of their clients that add sweeteners, perhaps the bitterness is better somehow as the base of a sugary drink.