So many GT mass-production sports cars engines taking from road car-sourced engines instead of built from scratch

[theriusDR3]

Here are the real reasons:
1. Cost efficiency
2. Saving time for modifications and tune-up

Any additions and your opinions?

I take example Honda/Acura NSX GT3 engines are using Honda J35 3.5-litre V6TT engines but Acura only for rebadging (not build from scratch) and also Chevrolet Corvette C8.R GTLM/GTE Pro engines are using General Motors LT6 flat-plane crank 5.5-litre V8 NA engines.

[Greg Locock]

Oh. Well have YOU ever made an engine (any engine) from castings, say? Do you have any conception how hard it is to machine a block from a casting?