You're taking a particularly narrow view of engine configurations, when there are many more variables at play.
The first is engine balancing; I6 engines (and by extension V12 engines as they are comprised of two I6 cylinder configurations) are inherently balanced and require less additional rotating balancing weight, or balancing shafts (common in I4 engines). For more information, see the following Engineering Explained video on primary and secondary forces.
Since each cylinder bank is essentially a straight-6 which is by itself in both primary and secondary balance, a V12 is automatically in primary and secondary balance no matter which V angle is used, and therefore it needs no balance shafts.
Beyond that, horsepower and torque are not simply two numbers; they comprise a function over the entire RPM range. Porsche, for example, is notorious for winning races with less peak hp/torque numbers by making higher hp/torque across a wider range.
The Veneno has an improved variant of the Aventador's engine; it's clearly in the Veneno because it would be more cost effective than redesigning the chassis to accept any other engine.
Lamborghini Veneno6.5L60° V12740 bhp235 kg
Ferrari 458 Italia 4.5L90° V8 562 hp ??? kg
As far as why Lamborghini developed V12s much earlier in their portfolio, I suspect a number of considerations went into that engine selection, including engine bay dimensions available, desired sound, balancing considerations, the notoriety that comes with a V12 engine, power band, and the success/failure of existing designs, among many others.
The primary factor, however, appears to be direct competition to Ferrari:
When Ferruccio Lamborghini set out to provide Ferrari with competition, he contracted Giotto Bizzarrini to design the engine for his car and, according to some accounts, paid him a bonus for every horsepower over what Ferrari's V12 could produce.
The engine was designed from the start to be a quad cam 60 degree V12 - as an intentional snub by Mr. Lamborghini of Ferrari's single overhead camshaft per-bank design.
Just to illustrate the narrowness of your view, "If V8s are ideal in every category, why does the Bugatti Veyron use a quad turbocharged W16?" "Why doesn't everything have a V8?" There clearly more variables to consider when designing engines.
Just for further consideration, the Ferrari 458 Italia is MSRP $239,000, while the Lamborghini Veneno is $4.5 million (based on the Aventador LP700-4, base price $441,000). Cost is clearly more of a concern to Ferrari on the 458 Italia than to Lamborghini on the Aventadors. This leads nicely into the advantages of a V12:
V12 engines deliver power pulses more often than engines with six or eight cylinders, and the power pulses have triple overlap (at any time three cylinders are on different stages of the same power stroke) which eliminates gaps between power pulses and allows for greater refinement and smoothness in a luxury car engine, at the expense of much greater cost. In a racing car engine, the rotating parts of a V12 can be made much lighter than a V8 with a crossplane crankshaft because there is no need to use heavy counterweights on the crankshaft and less need for the inertial mass in a flywheel to smooth out the power delivery. Exhaust system tuning is also much more difficult on a crossplane V8 than a V12, so racing cars with V8 engines often use a complicated bundle of snakes exhaust system, or a flat-plane crankshaft which causes severe engine vibration and noise. This is not important in a race car if all-out performance is the only goal. Since cost and fuel economy are usually important even in luxury and racing cars, the V12 has been largely phased out in favor of engines with fewer cylinders.
Just to demonstrate the 'bundle of snakes' headers of crossplane V8s, here's a racing exhaust setup on one.