In general, muscles should take more load than connective tissues. Even big strong ligaments and tendons (like the Achilles) do not have the absorption and range of the calf muscle they're connected to.
The connective tissues in your back are intervertebral discs, connecting your vertebrae together. They can absorb some impact, act as ligaments, and offer some flex and rotation. But they are not designed to allow you to pull 500 pounds off the floor.
When your spine is neutral, you are isometrically locking the erector spinae and abdominals, along with some other smaller helper-muscles.
When you then reach down to pick up that 500 lb weight for a deadlift, with that neutral spine, your intervertebral discs are receiving a load that is within their range to handle, but your back and ab muscles are where the real stabilizing force is coming from.
Rounded back lifts are important, and things like the straight-leg-deadlift (where your back will come out of neutral) are used by a lot of athletes. But it's also considered a potentially dangerous exercise:
Performing a movement with a weight or range of motion that is
significantly beyond what they are accustomed greatly increases risk
of injury, particularly with this movement, but that does not make [straight leg deadlifts]
a dangerous exercise when performed with common sense guidelines.
Those "common sense guidelines" rely on the fact that you're relying on weaker muscles and putting load directly on connective tissues. Provided you use great form and light weights, the essay linked above indicates you'll have good results. But that's a far cry from the proper and safe lifting form for heavy deadlifts, lifting heavy things around the house or workplace, etc.
Another good reason for practicing rounded back lifts is that often the real world doesn't afford us perfect anatomical conditions. Lifting a child out of a carseat, as an example. There's a bend, a twist, and the weight is fairly far ahead of you.
Also, isometrically locking your spine and doing things like a conventional deadlift does strengthen those muscles, in much the same way that a plank strengthens the abdomen.