People want to compete in PvP. In a competitive mode, even at low ranks, people care about having the best chance at winning so if some people are doing this hardcore grinding other people are gonna feel pressure to do that too.
It’s also 18 tasks per merc so it’s 40*18/28 which works out to just under 26 weeks which is half a year. People are not going to want to wait that long to be competitive in PvP. On top of that you can get day after day of lower reward tasks and/or tasks for mercs you don’t plan on using making things feel like they’re taking way too long to progress. If you’re just interested in casual play then that’s fine but to compete in PvP you need every edge you can get and waiting 6 months to do so isn’t reasonable when there’s a way to get that a lot faster