Here's why:

* Protostars are the earliest stage in the formation of a star. They form from the collapse of a giant cloud of gas and dust called a molecular cloud.

* As gravity pulls the cloud's material inward, the core of the cloud becomes denser and hotter. This process releases energy, causing the protostar to glow.

* The protostar continues to collapse and heat up until it reaches a critical temperature and pressure, at which point nuclear fusion begins in its core. This marks the birth of a main-sequence star.

So, while a protostar does glow, it's not yet a "true" star. It's still in the process of becoming one.