why isn't there a car with carbon fibre pistons and block?

Cost is the main drawback. Lubrication would be essential.

For those who believe carbon fiber is flammable, watch from 3 minutes to about 7 minutes as a piece of carbon composite structure heats up,

https://youtu.be/TxBj8R7XKe4?t=185

DIFFERENT materials for DIFFERENT jobs! CARBON is very HARD< but vulnerable to scuffing etc! PISTOSN today are HYPEREUTECTIC meaning that they are ALUMINIM with a mixture of SILICON in it! They CHANGE SHAPE as necessary to PREVENT pollution at different temperatures! Teh BLOCKS are ALSO mostly lined with CERAMIC or with SILICON and you should NEVER see any EMSISSION emitting cars or trucks anymore! The AIR is VERY CLEAN compared to decades ago! ALL CARS now are made just SSTART and GO with ZERO warm up time! I am SURE someone somewhAre is thnking the cAme thing YOU ARE! I keep wondering WHY we do NOT HAVE LASER equipped SNOW removAl trucks that VAPORIZE the snow and ice so we do not waste MILLIONS OF TONS of ROAD SALT every year and CONTINUAKLL:Y clean and rebuild ROADS afterward! OR even a LARGE HEAT GUN under side of a truck! There MSUT be a BETTER WAY to handle SNOW and ICE that is mroe environmentally FRIENDLY and KIND as well!

You've obviously never seen what heat does to carbon fiber.

Carbon Fibre cannot hold the Pressure or the Heat

Have you ever seen a Burnt Piston

Not the right application, the material depends on the type of force applied and what type of physics it has to withstand.

Heat dispersal may be its biggest problem.  Aluminum engines with iron piston sleeves is still the best. Steel rings are harder than iron so seating the  rings into the cylinder bore is needed to give a real tight fit between ring and cylinder walls. Aluminum is good for transfering heat away and it is light. Cheaper too than Carbon fiber..

That is my guess.  They continually experiment with different alloys of metal for everything.

Because at the end of the day, carbon  fibre is just very expensive plastic. ( It has to be bonded by a resin just like fibre glass).

Ever heard of a plastic engine?

That is plastic and flammable. 

heat , carbon fibre isnt heat proof , aluminium is just but carbon fibre isnt .

It's probably cheaper to use more commonly available materials at this point. Maybe one day