My TV is calibrated for rec. 709 input, not RGB (and frankly, so are most people’s, if they’re calibrated at all). Plus optical discs are a better archival format, and it plays back more reliably across all hardware players (some players have odd MKV limits, like they only play one audio track, or they won’t do DTS-MA if it’s on an MKV). Oh yeah, and it’s a good gifting format for less technically inclined (or if you have kids). And the box looks nice on the shelf.
Other than looking better, lasting longer, being more reliable, working better for friends and family, and the bling factor, I admit there’s not much reason.