[kossae]

This is likely the reason. I forget the article/post, but I recall reading about this on HN a while back. Basically the licensing for fonts across all platforms can become very costly, so developing a typeface in house becomes an appealing choice once your designs are in front of X number of people (commercial/web/print/etc).

[amptorn]

This is definitely the reason IBM invented Plex -- they have designers aplenty after the IBM Design initiative a few years back and they discovered that unfortunately Helvetica Neue costs money.