Oh boy. This is a politics forum but I don't want to turn this into a debate.
The OP's question title says "Why does Canada require mandatory bilingualism in all government posts?".
The body then says, "having fluency in English and French is mandatory (In government jobs)". It then goes on with some dubious Math and unrelated conjoined statements which are NOT "proven".
(disclaimer: I could not sit through the video, regret trying, would not encourage you to boost his views either)
To answer the title's question, it's the law and official government policy to make official government communications available in both official languages and for the right of individual citizens to conduct official Federal government business in either official language. You need bilingual staff to support that position.
It is also the law in the Province of New Brunswick. This is extended to a third official language in the territory of Nunavut. In short, it's the law and not subject to popular choice, convenience or shifting demographics.
There is plenty of opinion on the merits and value on the language policy (which I'll not get into), but it goes beyond "the law":
As a result of section 16 in the Charter, the Official Languages Act was amended and strengthened by the Mulroney government in 1988, leading the Federal Court of Appeal to observe:
"The 1988 Official Languages Act is not an ordinary statute. It reflects both the Constitution of the country and the social and political compromise out of which it arose…[I]t belongs to that privileged category of quasi-constitutional legislation which reflects ”certain basic goals of our society” and must be so interpreted ”as to advance the broad policy considerations underlying it.”"
In other words, bilingualism is a core tenet of what make's Canada, Canada.
It is also NOT a requirement for all federal civil servants to be bi-lingual, though it certainly helps prospective candidates
That contradicts the OP's claim,
only 17.5% of Canada is eligible for government posts
I happen to know many non-bilingual federal employees. If you have the skills required to fulfill the posting and are the most qualified candidate, you will likely get it. You may even be provided or be reimbursed French Language training.
To counter the unrelated conjecture,
... 83.5% of the populace is getting more uninterested in learning a secondary French or English language.
In Canada's Westernmost province (uni-lingual English), British Columbia (where the YouTuber is from, also never heard of him and his claim of " news commentator for television and radio"), it has been reported:
While general enrolment has declined, the number students in French Immersion has significantly increased. Enrolment in French immersion programs has increased by 89.4% since 2000–01
And they can't get enough teachers to fulfill demand, so demand outstrips supply.
To throw in a totally unrelated fact from the same report,
"full-time equivalent student enrolment in English language learning (ELL) programs increased by 17.4%".
Meanwhile, a news story reported in over 10% of Metro Vancouver (BC) schools, the majority of students are "English Language Learners", their primarily language being something other than English (and likely not French).