Why do our bodies have to decompose after we die?

We consist of organic matter; we’re basically [b]meat[/b], which spoils and decays.

There are so many ways to answer your question.
1. Because all organic matter decays.
2. Decay is necessary for recycling of finite earth resources. Ashes to ashes and dust to dust.
3. Once you stop breathing there is no oxygen in the body to keep cells alive quickly causing cell lysis( death).
4. Our bodies are actually under constant attack by fungi, bacteria etc but its only because we are living that we can fend them off.

Everything needs to decompose in this world,otherwise there would be no place on earth to fit all the dead bodies...

If you're talking about why there is a process of decomposition "biologically",I can tell you that the body of any living being human or animal,that once the blood circulation is cut off due to an incidence (death) there won't be electrolytes that give the body it's Oxygene preventing organs,blood,to rot.

To complete the cycle

it is the way of nature. all things decompose, unless removed from the biological world
microbes and fungus just doing their job

You can put it in a freezer and save the husk but nobody would want to be reborn into that ugly bag of water.

The left side of body goes to Alabama & the right side to Tennessee

no oxygen no blood flow bacteria its life ash to ash and dust to dust

Because we’re organic matter

It's how unclaimed or unpiloted living material travels through time.

Would you want to unearth faces when you start a new garden?

Not difficult to re generate if you can get a male and female to agree not to murder it.

Because our physical bodies are made of meat and meat rots.

To feed the maggots