The redesign they did years ago pushed a more Instagram like design. Things like chat and rPan are meant to keep you on the site interacting with other users in real time. Their new video player is just a TikTok rip off. They give away awards every day and started pushing award karma in hopes that you'll give them to other users. Reddit wants you on their site, interacting with other users in real time and building a better search goes against that goal.
If I'm looking for a new pair of boots, there's two paths I can take on the reddit site. I can search "best boots" or I can go make a post on /r/boots. If their search is fantastic and I find a post that's 2 years old, there may be some great information there, but I'm probably not going to comment. Even if I do comment, I'm probably not going to get a lot of responses. If their search is shit (intentionally or unintentionally) it pushes me toward making my own post and drives up their engagement metrics.
It would be a massive undertaking to improve their search and there's very little incentive. Reddit wants users who just want to browse and that's not what the users coming in from a specific Google search are doing.