Why is it das Boxen?

Boxen is the verb boxen used as a noun, and verbs used as nouns are always neuter: das Boxen.

More verbs used as nouns
Sense 1
das

Meaning

[1] Sport: a martial art that is practiced with the fists

[1] Sport: eine Kampfsportart, die mit den Fäusten betrieben wird

Examples

[1] I knocked out while boxing.

[1] Ich bin beim Boxen k. o. gegangen.

Declension

1 Singular Plural
Nominativ das Boxen —
Genitiv des Boxens —
Dativ dem Boxen —
Akkusativ das Boxen —

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary