Even though the signal gets low several times a second why our eyes are only able to see the bulb glowing all the time
It's due to the response of the eye and brain to rapid variations of light intensity.

Persistence of vision traditionally refers to the optical illusion that occurs when visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been described as &quot;retinal persistence&quot;, &quot;persistence of impressions&quot;, simply &quot;persistence&quot; and other variations. According to this definition, the illusion would be the same as, or very similar to positive afterimages. Source: Persistance of vision.

There is more in the linked article.
If you move the PWM display across your field of vision while looking straight ahead you should be able to see the strobing effect. I've also managed to see it during humming while looking at a multiplexed LED display. You need to vary the pitch of your humming until your eyeballs vibrate at a multiple of the strobe frequency. It's quite weird!