How could one single photon have only one direction? I mean at every point of its propagation the changing electric field creates a magnetic field in every direction (not just in the direction of the propagation). This would create an (elementary) wave source and it will propagate in every direction.
The photon is an elementary particle in the standard model of particle physics. As such it is described by a quantum mechanical wavefunction , in complex numbers, whose complex concugate square gives the probability density of finding the photon at (x,y,z,t). It is only characterized by its energy=h*nu, and its spin which is + or -1h towards its direction of motion. Thus the image you have of it as a source of radially propagating fields is wrong. The confluence of photons builds up the electric and magnetic fields of the emergent classical electromagnetic wave, but the classical wave format cannot be cut down to describe a photon.
Quoting from this answer by Motl (to a different question)
the wave function of a single photon has several components - much like the components of the Dirac field (or Dirac wave function) - and this wave function is pretty much isomorphic to the electromagnetic field, remembering the complexified values of E and B vectors at each point. The probability density that a photon is found at a particular point is proportional to the energy density (E2+B2)/2 at this point. But again, the interpretation of B,E for a single photon has to be changed.
Have a look at this answer of mine on a similar question.
Let's look at your question
"Why in case of light, the Amplitude doesn't seem to decrease when it
travels in vacuum(even though electric and magnetic fields from nearby
sources exists)?"
Perhaps the confusion is caused by the concept of a plane wave. Yes, indeed, a plane wave has an amplitude that remains constant throughout space. However, one never finds an exact plane wave in practical situations. Practical optical beams always have a finite transverse scale. You can think of the optical beam produced by a laser point. The spot of light that it produces has a finite size. As a result this beam will gradually expand as in propagates further and further.
In general on can have cylindrical waves or spherical waves, in addition to plane waves. The conservation of energy dictates that total power on a closed surface perpendicular to the direction of propagation must be constant regardless of far away that surface is (assuming of course there is no absorption of the optical power along the way). Power is the integral over the intensity over an area and intensity is proportional to the square of the amplitude. To satisfy this requirement the amplitude of a cylindrical wave must decrease as one over the square root of the radius of the cylindrical surface. On the other hand, for the spherical wave the amplitude decreases as one over the radius spherical surface.
Next question:
"It is said that from Maxwell's wave equation, light is a transverse
wave. So, when we draw a light wave, the changing electric field is
drawn mutually perpendicular to the changing magnetic field. The
Amplitude is the highest value of the function, but physically, the
value keeps on increasing and after reaching a certain point(the
Amplitude) decreases again up to it's negative value, where does the
light gets its energy to again oscillate from its negative Amplitude
to the positive Amplitude."
Some times the diagram could perhaps be misleading. The typical diagram showing the electric and magnetic fields represents the spatial shape of the fields as they are frozen in time. However, if one were to turn on the evolution of this field in time, how would the diagram change? It would shift in the direction of propagation. This is the basic property of a wave. If the frozen diagram for the electric field for instance is represented by a function $\mathbf{E}(z)$, then the corresponding expression for the electric field as it evolves in time is represented by $\mathbf{E}(z-ct)$. So we see that the function shift toward the direction of propagation (positive $z$-direction in this case) at the speed of light.
One can now use this evolution to see what would happen if we look at just one point in space and see what the electric field does as a function of time. So let's set $z=0$, then we get $\mathbf{E}(-ct)$. So we see that we get the same function but as a function of time and now it is inverted. The the electric field oscillates at any particular point in space.
The energy in the field is carried along with it. One can calculate the energy by integrating the power over time.
Hope all the issues have been addressed. Let me know if anything is still unclear.
Best Answer
There is the classical formulation of electromagnetism. In that, a varying electric field generates a varying magnetic field and the wave propagates because it has a directional vector that carries the power of the wave, the Poynting vector.
Dipole radiation of a dipole vertically in the page showing electric field strength (colour) and Poynting vector (arrows) in the plane of the page.
The electric field.
Look at the figure. It is a directional solution of the boundary conditions of the electromagnetic problem, and it carries energy and in the quantum mechanical representation with photons, momentum too.
No. The poynting vector , look at the arrows in the figure, is directional, and it is the direction in which the wave propagates. It is not continuous point sources.