It's not always future tense. Actually, the most common usage of -ㄹ has little to do with future tense. -ㄹ is just an "An ending of a word that makes the preceding statement function as an adnominal phrase."
As for why -ㄹ, over all other verb endings, is used for Hanja, we don't know for sure, but it's the way it has been for over centuries. Look at this Hanja dictionary from 1458 (初學字會) for example:
You can clearly see "콩 空 뷜"(Modern 빌 공 空), "통 通 아ᄑᆞᆯ" (Modern 아플 통 痛), which use verbs conjugated with -ㄹ.
Looking at the research paper "선어말어미 '-리-'의 기원", the -ㄹ ending was a nominalizing ending until the 16th century. The adnominal usage was done by adding the adnominal particle ㅅ after nominalization. So 뷜, 아ᄑᆞᆯ etc were considered nominal (noun-like) back in the 16th century. So it makes sense to use -ㄹ, it's akin to where English dictionaries use "to ..." for verbs.