Learn Swedish: Vi vågar inte överskrida kvoten på grund av att straffet är hårt. - We do not dare to exceed the quota because the punishment is harsh.

Because it introduces a full clause with its own subject and verb (straffet är hårt). If you dropped att, you could only follow it with a noun phrase, like på grund av det hårda straffet (due to the harsh punishment).

Is there a difference in meaning between på grund av att and just saying eftersom or därför att here?

They all mean "because", but på grund av att is slightly more formal and translates closer to "due to the fact that". It strongly emphasizes a direct cause-and-effect relationship, which fits well with formal rules like quotas and punishments.

Why isn't there an infinitive marker before överskrida, like vågar inte att överskrida?

The verb våga (to dare) acts like a modal verb in Swedish. Just like kan, vill, or måste, it takes a bare infinitive directly without att.

I see that hårt ends with a "t". Is that because it's acting as an adverb here?

No, it is an adjective describing the noun straffet (the punishment). Because straff is an ett-word, the adjective must take the neuter -t ending. It is common for learners to forget this agreement when the adjective comes after the verb as a predicative.