Because there usually isn’t one objectively “best” correction.

Soft proofing shows the compromises required when moving from one medium to another. Prints and displays differ in gamut, saturation, and dynamic range, and each medium can reproduce some colors or tones the other cannot. Once information falls outside the printer/paper range, the software has multiple possible ways to handle it: reduce saturation, reduce vibrance, lower contrast, compress tones, clip values, or use some mix of these.

Those choices produce different visual results, and the right one depends on artistic intent. One photographer may prefer preserving detail in highlights and shadows with lower contrast; another may accept some clipping to keep the image punchy. The software can convert colors automatically, but choosing which compromises matter most is a creative decision, not just a technical one.

So Lightroom can simulate the print and perform color management, but it can’t reliably know your preferred trade-offs. That’s why soft proofing is useful: it lets you decide how to adapt the image for that specific paper, printer, and look you want.