Because a website is where every decision a company has already made gets tested at once, in public. Positioning, pricing, category choice, who you sell to and what you say to them all surface on the site. The design and the build are rarely the hard part.
Five things have to be settled before design starts, and most of them are not website questions at all:
There is also no playbook, and anyone selling one is overstating things. Different buyers arrive at different points in the buying journey and want different things from the same screen, so no fixed rule about heroes, headings or button placement serves all of them. Experience does not give you a template. It gives you the ability to notice when a project is drifting and bring it back before it costs three months.
More on this in why B2B website projects are so hard.