Because u looked only the cheap one. Try much more expensive one, u will be satisfied. Like 10k for one bedroom apartment, if u live in shanghai.

haha...... you shall be thank ful at least you can get a bathroom and toilet in one apartment or flat. sometimes when you share it i think thats more horrible and sucks.
you know that chinese has a booming population almost 1.3 billion and these can not make leisure bathroom in their apartments as money talks.. if you go to a well develope place and society you will find a better condition but that will make your wallet wash away from BUCKs........
there can be many reasons for dirty bathroom in apartments... maybe it was shared by many people, or maybe the maintence guys are too lazy to do their job, you can think other possiblities

Some advice gleaned from my own experience: if your employer says that they'll find you an apartment, give them a list of everything you want it to have beforehand. My mistake was saying only that I wanted a seated toilet. When I viewed the place it did indeed have a seated toilet. Pity everything else was horrible; the place was absolutely filthy, there was no washing machine, mould all over the walls and the most sickening smell coming from the kitchen sink. As the lease had already been paid, however, it had to stick.

My response was to throw a fit, refuse to move in and threaten to cancel my contract immediately. Next day: the flat was cleaned up, washing machine installed, kitchen utensils bought and a rather nice potted plant plonked in the hallway.

Secondary advice: I live in a place very few foreigners want to stay. Knowing this and knowing how important a white face is to their business model I knew I wouldn't be refused. I don't suggest acting in a like manner in Beijing or any other place where you can be easily replaced.

Some places don't even have bathrooms. Even in Beijing's downtown.
You can't expect things to be exactly the same as back home.

Have lived in 2 apartments. The 1st one had a small bathroom by Australian standards that turned out to be a large one by China standards. It did have it's own shower module and hot water so was OK. The one I now live in has a smaller (less than 2mts x 2mts) bathroom and only has a hand shower so when you have a shower the whole room gets wet. It is a pain in the arse having to mop up every time you shower. Will be moving into a brand new apartment later this year and the bathroom is big enough for a shower recess and so it will be more of a comfortable Western style bathroom albeit still on the smallish side.

You also have to remember that it was not too long ago that locals bathed in communal facilities and many still prefer a trip to the local sauna to bathe where I live.  

Why they mostly stink is easy to explain, the toilet does not have a water trap to keep the odors away from you.  The drain on the floor connects to the one from the toilet, also the sink on washbasin (also no water trap).  This is done to save a few bucks.  On luxury hotels and higher type condos, they are provided, and there is no smell. 
But once bathroom is built, they are very hard to install, except for washbasin one.

At my apartment, what I do to prevent foul smell is to pour one cup of bleach down the floor drain, one a week at least, or when the smell hits me.  I also pour about 1/2 cup into bowl of toilet, let it seat there for about 8 hours, and flush it.  It is kept clean and does stop most of the smell from drain pipe under floor.

When you have untrained construction workers building everything for only a few rmb an hour, what do you expect?