Your site was built once, in a hurry, for a launch that has long since passed. Since then the business has added services, moved upmarket, opened new cities or changed who it sells to, and the site has stayed where it was. Visitors arrive from a referral or a search, see a home page that does not match what they were told, and leave without asking. Nobody inside the business looks at the site often enough to notice, because everyone already knows what it should say.
Why it persists. A redesign feels like a large, disruptive expense with no clear return, and the last one may have been exactly that: a fresh look, the same structure and the same result. Owners also fear losing the search rankings and links the old site has collected, and that fear is well founded when a redesign is treated as a design job rather than a migration. So the site is patched, page by page, until nobody remembers what it was meant to do.
If it stays unresolved. The gap between the business and its website widens every quarter. Each new service, market and hire makes the site a little less true, until it stops being something you can send people to. Competitors with a clearer site win the comparison your prospects make before they call.