Hang on a minute, planets DO twinkle. Have a look at some real footage of Mars for example and you will see it pulsate and change shape. This can be observed in ALL footage taken. In fact ALL of the planets pulsate.
I think you're confusing twinkling (changing in brightness when viewed with the naked eye) with whatever effect you're seeing through a telescope. The point is that even if a planet is kinda moving and pulsating through a telescope, it's not in such a way that the total integrated amount of light coming from the across the whole of the planet's disc changes all that much. Hence, when you stop looking through the telescope, the planet appears to be a nearly constant source of light. www.skyandtelescope.com/astronomy-resources/why-do-stars-twinkle/ ¯\_(ツ)_/¯
