1 It does the job well Monobook is a very reliable lightweight skin with simple but usable design and no uneeded flashy elements like most modern HTML5 websites, the only thing it's lacking is a dark mode for those long late night patrols.
2 It's an old skin you can still use Did you know that WM just released a new skin called Vector 2022? which deprected the very old Standard skin, but monobook is a classic skin and is likely to stay around for a very long time, it's the oldest skin that still looks like WP.
3 The Nostalgia Value To those like me who grew up in the 2000s, Monobook was the default skin of choice for WP, my brother used WP, a lot, and as such i saw this skin a lot of times and hence developed a lot of nostalgia for it, i'm so glad that i can relive this nostalgia instead of being made to use some modern minimalistic skin like most websites (WP has those as an option, which i personally choose not to use, i wish all websites were like that).
4 It's the familar old wikipedia Like most websites the look and layout of WP occasionally changes, but unlike other websites, at anytime you can choose to go back to the familar old layout and style, even if they release a new skin.
5 Just use it, you'll either think it's outdated or steeped in nostalgia and functionality.