Back to life
Pacing and immersion are important to the enjoyment of a video game and having to purchase in-game items almost always interferes with the experience like some promiscuous pop-up window.
Developers: If you really want to shatter the immersion of any videogame, just interrupt the play session and ask them to buy something.
Even the most basic games share the medium's value of escapism; an opportunity to get away from the stresses of life by throwing yourself into a virtual world. Breaking this illusion is not what developers got into the industry for, surely.
Some games get it right by either dealing with the buy-your-stuff part of the game by separating it from the core game (Diablo 3's real money auction house is an example of this) or by making the user interface elegant and streamlining the buying process. We consider these to be the minority.