Because I want to save the time I have. I use Bulma for 100% of my projects because I want spending priceless minutes of my own free daily time for the application _core_ instead of flexboxing viewports and debugging compatibility for numerous clients (and proving my point here).
I understand this sentiment, I really do... but this layout/design is so simple to make I think a framework would be overkill and unnecessary bloat. This is coming from someone who does css for a living daily so your mileage may vary.
