A few issues, the reclined seat will block the evacuation of those behind you as their space will be restricted. This space restriction may also cause injury to the traveller behind if there’s an abrupt stop (they’d get thrown forward and strike the reclined chair/person in it). Lastly, in the event of a more serious incident the chair design is such that it gives most strength in the upright position, up to 16G deceleration. It’s a certification thing.
Genuinely would love for someone in the airline industry to explain the impact of having a reclined seat for takeoff