I agree with this post. I spent multiple days on every application I've ever sent, tailoring it to the specific role, adjusting my CV etc. No way I could have applied for 100 positions, partly because there are only a few I am qualified for every year. I only ever applied for one PhD position and 2 research positions after my PhD and have always found jobs within a month.

I usually knew people (who knew people) who worked at every place I applied with. Reputation is extremely important, and by that I don't just mean professional but also how you get along with people, how helpful you are etc.

I've been involved in hiring multiple research assistants and postdocs. I've interviewed people who disqualified themselves by their unprofessional/ unlikable behaviour. One candidate asked the interview panel to participate in they research ("I'll send you the link later"), one candidate bragged about having pissed off multiple big names in the field. Others sent really unprofessional emails to our PI before the interview. Remember that any interaction with the organisation you're applying with is important. If HR, receptionists, students etc have a bad feeling about you, they will let the PI know.

I've interviewed multiple people who were qualified on paper but did not reply to interview questions adequately. You really have to prepare for interviews and have answers ready for every question that you can anticipate.

Finally, everyone I've known who could not find a research position after months/years of applying had a reason. One person admitted to answering "I don't know" in interviews a lot. Another person acted unprofessional during a lab visit to us shortly before applying for a position in our lab.