With a hybrid, there is a complex efficiency trade-off between the power and charge capacity of the electric components, and the cost of dragging them around with you when the vehicle is running on its internal combustion engine. The designer takes a (hopefully well-informed) view of the likely spectrum of journey distances and road types that the average owner will require, and then optimises the electrical system for overall vehicle lifetime efficiency. (Note: if you are very far from that average owner profile, a hybrid may not be the right vehicle for you).
I'd guess that the electrical system is aimed at use in stop-start urban traffic, and for shorter commutes. Internal combustion engines are at their worst in stop-start conditions. Lighter weight trumps blistering e-acceleration (needs heavier motors) or long electrical range (heavier batteries). Those requirements are satisfied by the other engine. You have the option to engage it even in city traffic, if that's your (energy-wasting) style.
You can always buy an all-electric Tesla ... but you have to be sure about being able to recharge it before its batteries run empty, and even if there's a charging station on route, recharging it is less quick than refuelling. It'll be a fair while before an all-electric car is any good in rural parts (where the locals will tell you that the electricity suppy is less than totally reliable, and many of them own a petrol generator, just in case. Thought -- a multi-kilowatt inverter accessory for a hybrid car might open up the rural market? )