The answer to 'why is my mulch weeds' is that weeds can easily grow through mulch if the mulch layer is too thin or if the mulch has broken down over time. Mulch is meant to suppress weed growth, but it needs to be maintained at the proper depth to be effective.
Common causes of weeds in mulch include not applying enough mulch initially, the mulch breaking down and becoming too thin, or the mulch being disturbed or moved around exposing the soil underneath. Proper mulch maintenance is key to preventing weeds from growing through.