That’s why. The joint between the cabin top and the hull is a) full of roots; b) full of holes from all those fittings, and c) lacking in essential sealant.
What to do?
Scrape it out. Squeeze in gunk. Bolt together. Fill all unnecessary holes. Fibreglass the joint properly. Spread more gunk. Place the new deck sections. Bolt them or screw them down. Put the Toe rail back ( after spreading more gunk). Put another layer of fibreglass resin over the whole thing. Get new posh “plastic teak planking” put on. Fix all the fittings all with lots of gunk.