Because it was considered normal, really.

The practice of boning or cording the bodice of an underdress to give support to a woman's torso arose in Europe around 1500, and by the end of the century it was most common in the upper classes for the boned bodice to be a separate garment detached from a petticoat. It had taken on a firmly conical shape by the mid-sixteenth century, as can be seen in this portrait of Mary Fitzalan from 1565, and flattened the bust somewhat in creating a smooth front for the gown. This general shape continued to be fashionable, but early in the seventeenth century a much lower neckline came into use, as I discussed in this previous answer. The depth of this neckline could show the curves of the breasts, and even possibly the sternum between them.

While separate stays went out of fashion shortly after this, boning being put in the bodice of the gown instead, they returned near the end of the seventeenth century, when a form of gown called a "mantua" started to gain popularity. This was actually a radical shift: mantuas were largely fitted through pleating and not very difficult to construct, and proved to be the in seamstresses needed to push through the monopoly that tailors' guilds held on legally sewing for a living, since the logic of barring women from tailoring was that the construction of boned bodices and stays were too difficult for them. The mantua had no boning at all and slipped on like a coat, requiring stays to be worn underneath. Early in its development, it was frequently worn open over the stays and petticoat, just fastened at the front of the waist, which created a low and square neckline. By the 1710s or 1720s, it was more common to cover the visible area of the stays with a separate piece of fabric (called a stomacher), but this had no effect on the shape or depth of the neckline, which remained the same throughout the rest of the eighteenth century.

Women did cover their chests with neck handkerchiefs and tippets during the day, with short cloaks when it was cool, and with ruffles sewn into the neckline of the shift or gown, but on the whole the shape given by stays and left uncovered by the gown wasn't seen as immodest, because it had been like that for generations.