system
Given the following post, respond as if you're commenting on Reddit:
human
[FMA] How does Alchemy work?

We see three types of alchemy throughout the series.

1 - Turning one thing into another, such as stone into gold. In fact this is one of the rules of alchemy not to do so as it adversely affects the economy.

2 - Synthesis and Decomposition, such as combining the elements of a human body to create a human doll, or parts of a radio to repair it. Or decomposing water to create a highly flammable gas to ignite.

3 - Transformation of matter from one state to another, such as freezing or melting or boiling.

My question is, why can't they do this more often?

There are various points at which is might be simpler to do so. 

Such as while inside Gluttony with all the blood, why did they have to eat Ed's boot? Why couldn't they take blood, transform it into something edible like stone into gold?

Ed travelling through the desert and having his automail heat up. Why couldn't he just ask to stop every few hours, create a pool of cold water and dunk his automail in?

And most importantly, the greatest question of all "What could equal the value of a human soul?" Couldn't they just take a bunch of something, transform it into something valuable and just transmute tonnes of it in exchange for a soul? Or even easier, just transmute it to create a leg and/or arm, which we have seen is the price of transmuting a soul to be able to bond with armour?
gpt
Organic substances are incredibly complex to make and very few alchemists can manipulate them, let alone create them from scratch.

Different Alchemists specialize in different forms of alchemy Ed cant manipulate sand very well.

It is incredibly difficult to put a price on a human soul in order to make a fair exchange, this was pretty much the main point of the entire series.