There could be many reasons. Off the top of my head:
* Google is not a devices company (disregarding recent forays into the the high end smartphone market), but a services company. It's not in their DNA to this, and it doesn't fit their (current) business model. The don't have the required hardware competences, nor the required sales or distribution organisations.
* Even if Google had the required competences to do this, it's highly doubtful that it's a worthwhile pursuit. The "pro" market is not big, its probably not growing, and the competition is fierce. To the extend that Apple neglects this market, its probably because the ROI is too low. To make hardware that seriously challenges Apple (and other PC manufacturers) requires large investments.
* It doesn't support Google's other businesses. Google is already present on all the existing platforms. They are not going to sell more ads doing this.