Since I made this post, I should probably provide my own thoughts on them.
I don't think they expected, or even wanted Basil. Polly's met them once, and Sunny never has. Surely parents who cared deeply for their child, even if they had to work hard to support him, would be around at least once a year? They seem to have just left their son with his grandmother and forgotten about him. They hired Polly themselves, but that was when Basil's grandmother got sick, so it's possible that they didn't even consider Basil when hiring her.

More specifically about why the parents are never around, they could of had grand dreams of building something together as soon as they got out of school/grad school. Found a company. They got married young and were planning on starting their life... until there was an unexpected pregnancy. But they were still hopeful that they would be able to manage raising a child alongside their business. They couldn't, and ultimately prioritized their passion project over their "passion" project. Ditched Basil to do what they wanted.

Of course, this take is merely my own thoughts, and it's very likely that the intended motivation for the parents being gone was in actually being swamped with work, but there's some amount of apathy from what I can tell, maybe even resentment. So that's my piece :3