why dont many people put flexi exhaust pieces in there exhaust system on kit cars?

i have an 1800 zetec and plan to put a flexi in the system, surely its better to have one than not to?

You want if you want but it isn't really required.
Flex couplings only came into widespread use with transverse engines.
Because of the torque reactions transverse engines have a lot of angular movement on their mountings which make it more difficult to design an exhaust system.

Thanks mate, they actually makes a lot of sense! I never thought of it like that! May still fit a small flexi

As a rule, if you solid mount the engine, you solid mount the exhaust. If your engine moves on its mounts, you want the exhaust to move too. If your engine moves but you don't want the exhaust to move the same way, you add flexi joints.

In many cases on a kit, by the time you've got headers, cat and silencer there just isn't enough length for a flexi.