I noticed that the depolarization in the ventricular action potential (bottom image) is just a sharp spike, with no local potential that gradually achieves the threshold potential (like the top image, depicting a neuron). What is the physiological/biophysical reasoning behind this?
Image sources: Top: http://humanphysiology.academy/Neurosciences%202015/Chapter%201/P.1.1.2%20Action%20Potential.html Bottom: https://aneskey.com/chapter-57-cardiac-muscle/
The time scales in your two diagrams are different by a couple orders of magnitude. There's something like a "local potential" in the second trace, it's just so steep you can only see that its height is sufficient to reach threshold, not see the duration.
The thing labeled "local potential" in your first picture, depicting a neuron, is in my experience more often called a "receptor potential" or "generator potential", or in the context of inter-neuronal communication a post-synaptic potential. However, it is in fact "local" if you think of the full geometry of a neuron that has a long axon. Specifically, if you record in an axon you're not going to really see this "local" potential, caused by current through some sort of receptor channel on the dendrites or in the vicinity of the soma. Instead, any small segment of axon is getting a really big input from ions flowing in the cytoplasm inside the axon, very quickly reaching local threshold and accompanied by further opening of channels and propagation of the signal. You could get the same effect if you just produced a really strong input at the soma, for example if you use your amplifier to deliver a stimulus in a cell you have under patch-clamp (which is how you get these recordings in the first place).
The specific diagram you show doesn't have a scale on the time axis, or you've inadvertently cut it off, but for an approximation, a typical action potential width in a mammalian neuron is around 1 ms. Have a look at the time scale of your second image, though: 100s of milliseconds. Note that I wouldn't call this a "ventricular action potential" but rather a "cardiac action potential". Cardiac action potentials are sllloowww.
Cardiac muscle cells are also not connected to each other through chemical synapses, like typical neurons where a neurotransmitter diffuses across a synapse to reach a receptor on the post-synaptic cell; instead, they're connected via gap junctions. These are protein "shunts" between the intracellular space of two neighboring cells, that let them share voltage changes directly, almost as if they are all one big cell. That's what's happening here. There is a potential before the cardiac action potential that's triggering the whole thing, but it's a very strong signal similar in dV/dT to the action potential itself. If you expanded the x-axis a lot you might be able to see some inflection there, but it's not something you're going to see with a scale on the order of 100s of milliseconds.
