Trading is actually an exact science
Just compare so called tick data backtesting on mt4 and mt5 and you will see the difference
The more you understand market mechanics (how it works), the better you understand why it is NOT an exact science:
What is the market?
A market consists of buyers and sellers with a commodity (product) of exchange.
Who are the buyers and sellers?
Humans (retail traders), algorithms/programs, banks (with humans and computers), governments (monetary intervention), brokers, (more?)
What influences the market?
Government policies (eg. interest rate decisions), economic data, government stability (leadership), banks, etc.
A science experiment is predictable: no matter how many times you conduct the experiment you will always get the same result.
A market experiment is unpredictable: think of the 'butterfly effect' (https://en.wikipedia.org/wiki/Butterfly_effect
...since humans are the unpredictable variable.