If I had a nickel for every time that question has been asked...
Now days, a lot less time is spent on the tracking/recording phase of an album. Most artists are on the road, recording their vocals in the bus, or on the hotel room. Perhaps they are recorded in sub-par environments, causing more work for a mixing engineer. While a lot of things can be fixed after the fact, this puts more strain on the mixer. Hours of work can be spent just to clean up, enhance, replace, or re-do parts.
The biggest question is why? Is it a push from labels? Or perhaps from consumers? The idea of releasing music constantly is putting a strain on the engineers. While we all do the work, taking extra time in the tracking phase of the song would eliminate a lot of time spent on the song. Budgets are not what they used to be, we all get it. As much as I would love a band to walk in with a $50,000 budget, it just isn't happening anymore.
However, with lower budgets and streaming services, it has leveled the playing field for artists. While the bigger labels can still afford to send their artists to the big studios, mid-sized and small studios have a chance to tap into the market. Pricing is always a struggle, so it comes down to providing excellent service. While we would all love to own / work in the Balckbird and Abbey Road type studios, there is only so much business out there. There are great smaller tracking studios out there, which can provide polished tracks for the mixing engineer.
A lot of bigger mixers have started working for lower rates as well. With this shift in rates, having your music recorded at a local, project studio, and mixed by a grammy award winner is within reach. Spending the time to get it "right at the source", so it sounds great as a raw recording helps the engineer take it that much further. Mixes can only be polished so much. Keep this in mind when you are planning your next album. Spend the time to record it properly, this will net you a better album in the end. Your mixing engineer will love you for it.