Because of the law of conservation of energy.

Yes, you need to collect energy from nuclear reaction, otherwise it's a waste of time.

Mainstream scientists will tell you that there is no easy way to achieve this, that you need very large complex reactors to get nuclear fision, etc.

Not true. Low Energy Nuclear Reaction (LENR) is possible and many people have done it (including myself).

Stanley Meyer, the inventor of the car that ran on water, used a small reactor to extract energy from hydrogen atoms. He was assassinated. But before that, his invention was rejected for "radiation emissions", even when the emissions were lower than a regular cellphone today.