Providing replacement bus service is a great idea. But doing so is going to be a lot easier on a weekend than on a weekday, simply because weekdays are far more busy.
If 700,000 people use Metro on a weekday and 200,000 people use Metro on a weekend, finding enough buses to accommodate the 200,000 is going to be easier... especially because a lot of regular bus routes don't exist at all on weekends, freeing up those buses (that would otherwise sit in a garage/yard somewhere) to be used for the rail replacement service).
I understand that it sucks to "always" schedule rail disruptions for the weekend, but it makes a lot of sense logistically: fewer people impacted and more resources (empty buses) available for replacement service.