Because no matter how prepared I think I am, there's always at least one item on the ballot I didn't expect to see.
I can sit down at my computer and look up everything on my ballot and make better informed decisions in the comfort of my home. I can lean over and ask my spouses opinion on what they're voting for and we can decide what we want to do. In the end we each make our own decision, but at least we can discuss it.
I have a QR code on my envelope that shows that I can track my ballot from end to end and ensure that it's where it is supposed to be. If it never arrives, I can go vote in person and have a provisional ballot to ensure I didn't vote twice.
Most important for me is being able to simply read the ballot and research candidates at my leisure. Not sure about failure points, my state has used mail voting for many years without issue and if Trump's lackey makes ballots go missing I will know because I can track my ballot on the state's website.
A really important point is some voting machines are 'ballot marking devices' which give you a human readable paper ballot with your choices (good) however the actual counting is done using a barcode or QR code (bad). There is never a good reason to prevent voters from verifying their ballot has their intended choices, yet some areas still insist on using these absurd machines.
All my ballots have been received and counted so far just fine. I'll go to the polls if there is a problem but until that changes, I'm going with what's convenient.
People who start from an illogical assumption and then expect you to argue against it confuse me. "Failure points"? I'm guessing you're young and don't mail things often, but they basically don't happen. Like, ever. 999/1000 times, it just gets there, they all do. The same could be said of risking a malicious person collecting the polls at a drop-box, or it could get dropped in the election day chaos.
As for why they might not want to go in person? That's literally the easiest option, there's not really a middle-man. My state sends the ballot in the mail (yes, I've always gotten it). Then we can leisurely fill it out at home, privately, with all the time and resources to research ballot measures, etc. Then just drop it in outgoing mail at home or work and it'll just get there. Again, never even heard of anyone ever having issues. And you can track it and submit a provisional ballot in case something does happen to the original one.
You can actually track mail in ballot, so if there is a failure, there would still be time to fix. It not like that the vote failed and people won't know.
Because people have jobs, and when people are not working, the last thing the want to do is wait in ridiculously long lines.
On top of this, with mail in ballots you have time and privacy to research all available candidates and proposals. You're generally not allowed to use an electronic device in a voting booth and researching in line might be illegal in your state if you don't have a way to hide what's in your screen; so it's nicer to actually get informed and vote instead of just the disasterous and genuinely anti democratic idea of "vote blue/red no matter who."