As you say, the more expensive models are better most of the time.
Since we can't easily predict which model will actually be better for a given question at the time of asking, it makes sense to stick to the most expensive/powerful models. We could try, but that would be a complex and expensive endeavor. Meanwhile, both weak and powerful models are already too cheap to meter in direct / regular use, and you're always going to get ahead with the more powerful ones, per the very definition of what "most of the time" means, so it doesn't make sense to default to a weaker model.
Since we can't easily predict which model will actually be better for a given question at the time of asking, it makes sense to stick to the most expensive/powerful models. We could try, but that would be a complex and expensive endeavor. Meanwhile, both weak and powerful models are already too cheap to meter in direct / regular use, and you're always going to get ahead with the more powerful ones, per the very definition of what "most of the time" means, so it doesn't make sense to default to a weaker model.