Learn Spanish (Latin America): Ella se esfuerza por tener más voluntad.

Because the person doing the striving and the person having the willpower are the exact same. When verbs expressing wants, efforts, or emotions share the same subject for both actions, Spanish uses the infinitive. If she were pushing someone else to have willpower, it would trigger the subjunctive.

Why is there a por before tener? Wouldn't para make more sense for a goal?

In Spanish, many verbs are permanently married to a specific preposition. The verb esforzarse requires por before the action being attempted. You just have to memorize esforzarse por as a set phrase meaning 'to make an effort to' or 'to strive to'.

Why is this verb reflexive (se esfuerza) instead of just esfuerza?

When it means 'to strive' or 'to exert oneself', esforzarse is a pronominal verb, meaning it needs those reflexive pronouns (me, te, se, etc.). If you drop the pronoun, esforzar usually means to physically strain or force something else.

Is voluntad the exact word for willpower?

Yes, it is often used on its own to mean willpower, though you will also hear the longer phrase fuerza de voluntad (literally 'force of will'). It can also refer to a person's general will, consent, or desires.