If it's not immediately clear why, imagine that you're a fantastic developer who can get any job you want. How many jobs do you apply for per day? Would you use a web app that automates applying to jobs, even if it worked really well and were free?
I spend at least a few hours writing job posts. Why shouldn't I expect the same?
I am not asking for paragraphs, but show me you are interested. I honestly spend hours writing and reviewing the jobs I post and it takes probably 100+ hours total from a team to hire a person. We want to hire you, but you've got to help us. :)
For me "Hey, I saw your post for [...]. I thought this looked great for me because I have worked on [...] and I am super excited to learn more about [....]" means absolutely nothing. Frankly, I do NOT know you or your company and in a job description there is nothing about the team, culture etc. But I have no other option just to make something up, because otherwise no one'll hire me.
Also I don't think one has to spend a few hours to customize the resume. Make one for each industry you are considering and spend 5-10 minutes tweaking it for the employers you really care about.
Even at Microsoft this was often the case - of course there was a generic template, but you would put specifics to the role.
If that annoys some people, well, that is why you apply to more than one job.
I mean, how much time did you spend approaching each developer individually? Since you used an ad, I'm guessing zero. I'm not saying you should have tailored your ad to each person, but why do you care if they burn time for you, rather than evaluate them on what they can do for the company?