Why is the verb 有 negated by 没 instead of 不 ?
Is there some historical reason or was it just a strange accident ?
有(have) is opposite of 没有 (don't have/ have not). 没 short for 没有
沒有工作 = 沒工作 = don't have job/ work
沒有去工作 = 沒去工作 = have not gone to work/ do not go to wprk
是(positive/ is) is opposite of 否(negative) or 不是 (is not)
是否合適？= is it suitable? or is it not suitable = is it suitable?
是不是人? = is it a human? or is it not a human? = is it human?
不合適 = not suitable
不是人 = is not human
Difference between 不 and 沒
不 = not; negative
沒 = have not; don't have
不工作 (not working)
不 here implies "refusal" of a verb
沒工作 (don't have job)
沒工作 (have not work)
沒 here implies "absent" of a noun or "negative" of a verb in past participle form
Since 有(have) implies "present" therefore we use 沒(don't have) for its opposite form "absent" (沒有)
We say 不是 instead of 沒是 because 不 here implies "negative", thus negative of 是(being) became 不是 (not being).
If we say 沒是, it would imply 1. "don't have being" ("being" in not a noun here) or 2. "have not being" (it would be in past participle form), neither is correct for expressing "not being"
This is a knottier question than it first appears. The answer is hard to summarise, but it seems to be related to the special status of 有 and 無 from the beginning of Chinese.
In the Old Chinese of the oracle bones, there seems to be fairly neat system of four. 不, 弗, 毋, 勿, where each of these is a combination of p-type vs m-type, reflecting non-modal (uncontrollable) vs modal (controllable / volitional) respectively; and vowel-final vs *ɯd final, which reflects stative/eventive/passive vs not. There was also 非 as a straightforward merger of a p-type with the verb 惟／唯／維.
Discussions of oracle bone Chinese often keep 亡 and 無 to one side, as they are verbs with negative polarity, and they replace 有 in negative sentences, rather than negating it. [Look in the 詩經: 亡 and 無 rarely paired with 有, and the two times 無 is with 有, it has a "futurity" about it, similar to modern Mandarin 不要 or 不會] We thus see that even from the time of Old Chinese, 有 was already a bit "special", different from other verbs. Compare Tibetan མེད med, the negative form (and not particle / prefix) of ཡོད yod, to be / exist (used in the construction for have).
According to Chappell & Peyraube (2016), 沒 was used before the Yuan, and only appeared with 沒有 from the Yuan onwards (specifically by 1350). It had been "semantically extended" from its root meaning of "sink" via "disappear", replacing 無 and then "extending to an aspectual negator or a marker expressing a perfective negation".
We see that in southern varieties of Chinese, negative equivalents of 有 (rather than negative adverbs) still exist: e.g. Min 無 (Southern Min POJ: bô, Fuzhounese BUC: mò̤); Cantonese 冇 mou5 (with a different tone to the now literary 無 mou4), Northern Wu 嘸沒 (Shanghainese hhmm meq). Interestingly, Xiang varieties generally use 冒得 instead of a cognate of 無 on its own for the negative of 有: an example of suppletion; 無 is still used as a perfective with verbs though.
So there is just something about 有 and 無 that became amplified with its use in the "perfective" through Middle Chinese.
Our moderator @songyuanyao has already provided a good link, here I make some additions:
没 is the opposite of 有, which implys the objective absence, while 不 is mere negation.
Is 不有 valid?
Yes! But it appears in classical Chinese, and idioms originating from it.
But! A very important thing is that 不有 is not closely combined to mean 没有 today. 不 is just a negative word negating the whole in the ().
You might ask: 我不（有工作）seems valid, too! But when lacking classical context, it appears very harsh to hear.
So if you write something like “明，年已弱冠，学已通博，反闲娱于家宅而不有业。”（小明年纪也成年了，书也读出来了，却闲在家里玩没有工作。）I think it's Okay QwQ.
没 denotes the sense of non-existence, while 不 is a general negation. The other way to think of it is that 没 is the short version of 没有. In other words, you can replace 没 with 没有 when 没 precedes a verb. For example, 他没来=他没有来; 门没关=门没有关;etc.
不 is used with other words(except for 有) such as 是 to form the negation. E.g. 这不是我做的; 我不管; 我不说;etc.
Furthermore, compare 我没有说 and 我不说. 我没有说 means the fact that I have said it is non-existent, that is to say, I haven't said it. 我不说 means I won't say it.
Hope this clear.