This is a question that people have seldom ever asked. In the English language, past participles are verbs that usually end with "-ed" (or "-ore" for those whose present participles end with "-ear"). But bear seems to be an exception. It not only has bore as a past participle, but also borne. That doesn't make any sense. At first glance, people may mistake borne as either an adjective or adverb when it isn't really either.
To be clear, it is not standard to use "bore "as the past participle of bear. Bear has borne as a past participle, and possibly also born, if you consider the "be born" construction to function in present-day English as a passive form of "bear." (To me, the connection between "be born" and "bear" seems more a matter of history, not so much an active connection.)
Many past participles end in -en, -n or -ne. Past participles formed with a suffix containing /n/ are considered irregular today, but they come from the historical conjugation of "strong" verbs, which are a large and important category of Germanic verbs.
Most relevantly, the past participles of tear, wear, swear are torn, worn, sworn. Some other past participles not ending in -ed are done, gone, eaten, broken. There are many more examples. So "borne" is not very exceptional in not using -ed. The fact that it takes the spelling -ne rather than -n (if we leave out the "be born" construction) is more surprising.
All past participle forms, whether formed regularly with -ed or irregularly in some other way, can be ambiguous with adjectives. Many adjectives have the form of past participles: for example, frozen, excited, dejected, disappointed.
“Bear” has two past participles, depending on the meaning you want. We use “borne” for most meanings, but “born” for passive constructions referring to birth:
"carried, sustained, endured," past tense and participle of bear (v.) in all senses not related to birth. Distinction between born and borne (q.v.) is 17c.