Why can an intransitive verb has a passive meaning in Japanese?
This question sounds like a pseudo-problem because there's actually no "passive meaning" in the world. It seems to be true that you're so proficient in English (unlike me) that you can sense "passive meaning" as like Germans sense the gender of a noun (but occasionally muddle them up in hazelnut cream) or Japanese know the correct counter word to use, but it's only a language-specific matter that can never be generalized. French disagrees with German in gender, so does Korean with Japanese in counters. Likewise, using passive or not is really depends on each language (below is German):
Ich war erstaunt. (lit. "I was astounded.") vs. I was astounded.
Ich bin erschrocken. (lit. "I've startled.") vs. I was startled.
Ich habe mich gewundert. (lit. "I've surprised/wondered myself.") vs. I was surprised.
You can see German, a very close language to English, has three verbs each takes different construction where English uses passive in all cases. "Passive meaning" just doesn't exist.
So, the meaningful part in your question is: why we form phrases as 「財布が見つかった」 instead of 「財布が見つけられた」?
The answer is, Japanese passive ("～られる") is used when you emphasizes external interference. It's not an convenient tool to make intransitive from transitive. In other words, it sounds like "the wallet was found by someone" rather than "the wallet was found". Since in most cases Japanese has proper intransitive counterpart for each transitive verb, you should try using them first, unless you believe whodunnit, howdunnit, or the fact it was done by something else (e.g. it was intelligently designed!), are relevant information.
Related answer: Examples of when passive form in English takes active/non passive form in Japanese
Further reading: 自動詞文と他動詞の受身文（中上級を教える人のための日本語文法ハンドブック） (Japanese)