Because engine balance is about balancing the first and second order vibrations caused by the moving masses of the reciprocating components like the pistons and conrods by using weights on the rotating crankshaft. 

Crankshaft flexing is also the reason inline 8s couldnt rev terribly high even though they were naturally balanced without a balancer shaft. 

With a flat plane V8, balance is often ignored entirely, but they can be balanced by using balancer shafts which are completely separate elements from the crankshaft itself. They add overall friction to the engine, but they dont add weight to the crank. Friction and packaging are why they're often omitted in sports cars.

V6s and crossplane V8s have fairly heavy crankshafts to make them balanced. Inline 4s are either unbalanced or need a balancer shaft. Theyre usually left unbalanced for performance/race cars, balance shafts for a lot of road cars.

Inline 5 can rev but its not totally balanced. Inline 6 is perfectly balanced but has a long crank with 7 main bearings so more friction. V8s have 5 mains and cross vs flat tradeoffs. V10s are minorly imbalanced but can have light cranks and be run without shafts.

V12 is the ideal config for free revving and perfect balance.

But its important to consider firing interval for smoothness as well. With even firing intervals, more cylinders = smoother power delivery due to less or no negative torque on the crank.

You need an 8 cylinder engine to have no negative torque on the crank at all. 6 cylinders do go barely negative but its not significant. With 4 cylinders or less there's considerable negative torque dips.

Something i didnt mention was also exhaust pulse scavenging which is easier to do with flat plane V8s than cross.