Why fry onions without oil and then add oil

It's not so much the taste as the texture. If they haven't been sauted first, the onions stay relatively crunchy during the rest of the cooking. The same is true of the garlic, but you'd usually have cut the garlic into much smaller pieces so it doesn't take as long to soften up, hence kicking the onion off first and adding the garlic a bit later.

The flip answer is that sauteed food will taste good, and the microwaved version you're describing will not.

A more useful answer, however, would consider physics and chemistry. Microwaving excites molecules, resulting in heat, presuming an adequate supply of water molecules to excite. Sauteeing conducts heat from the heat source to the food by way of the pan.

Microwaving falls somewhere between radiant heating (like an oven) and steaming, though the results from conventional methods are generally superior. Sauteeing can transfer enough heat quickly enough that desirable effects like caramelization and similar effects like browning are effected with the help of fats and sugars (often referred to as a Maillard reaction), which produces volatile compounds that are very aromatic and contribute a lot to our perception of flavor. It's exceptionally hard to create the same effects with a microwave.