If religion is supposedly so liberating then why so many strict rules?

At best, religions can be a path to liberation. As the Bible says:-"You shall know the truth and the truth shall set you free" (John 8:32)

Or in the Buddhist Theravada texts:-

[i]So this holy life does not have gain, honour, and renown for its benefit, or the attainment of virtue for its benefit, or the attainment of concentration for its benefit, or knowledge and vision for its benefit. But it is this unshakeable deliverance of mind that is the goal of this holy life, its heartwood, and its end.[/i]

As a path, at the beginning, often much self-discipline is required. Therefore rules.

Unfortunately it can all become corrupted. Discipline and rules, instead of becoming a means to an end, become the thing itself.

Meister Eckhart:- "They do Him wrong who know God in just one particular way - they end with the way rather than God"