Just like light, some materials absorb microwaves, some materials reflect microwaves, and some materials transmit microwaves. You may have noticed this: when you put food in the microwave, some parts of it get very hot, and other parts of the food are still cold. Some of this is due to the uneven distribution of microwaves in your microwave oven, but this is also due to the fact that certain materials just don't absorb microwaves very well. For example, put a mug of water in the microwave--the water might be boiling hot, but the mug will be cool enough to hold in your hands.
One of the reasons why microwaves work so well at heating food is because food contains water, which absorbs microwaves. If you made your house out of water, your WiFi wouldn't work very well. The same thing happens if you use a lot of reinforced concrete--the rebar reflects microwaves, and you get bad WiFi reception. But most homes have walls made mostly out of air, with a little bit of gypsum, paper, and paint in front of the air.
If you ran a microwave oven with the door open, it would send a "signal" that would travel pretty far. You would probably piss off the neighbors. Maximum WiFi power is around 1 W, so 1500 W of power would travel ~40 times as far.
This is actually the reverse of how the microwave oven was invented--an engineer named Percy Spencer was working with a radar set, and noticed that the radar set was melting his candy bar.