<p>Time and depth.</p>
<p>The coin of the realm in research is publications. For fields where graduate students commonly work with and collaboratively research with Professors, the Professors will always opt for funding PhD students. There is a steep learning curve for really contributing inputs to peer-reviewed publication quality research outputs. There is a short window once a masters students gets up and trained to generate collaborative research publication. Phd students extend that window 2 or 3 years, get deeper human capital investment (learning as they go), and can make more sustained research contributions. The PhD student is also more directly that signalling that they want to pursue publication quality research.</p>