Why do military jets seem to always take off using the afterburner?
Military aircraft takeoff with afterburner when it is safer to do so with an exception- carrier pilots always light their cans while taking off (and also when the pilot wants to show off, but lets leave it at that).
The main consideration for using after burner in operational land based aircraft is the available runway length. When the aircraft is loaded heavily with ordnance, the runway required may be more depending on other factors (like OAT etc). In this case, it is better to go for afterburner takeoff. For example, Air Force Instruction 11- 2F- 16V3, F- 16 Operations Procedures specifically states:

3.6 Takeoff
3.6.6 Make an afterburner takeoff anytime the computed MIL power takeoff roll exceeds 50 percent of the available runway.

The same is true for other aircraft like the F-15 too:

3.7 Takeoff
3.7.2 ... For single ship takeoffs, if the  single ship computed  military power takeoff distance exceeds one-half of the available runway, takeoff using afterburner.

Another reason the pilots do it is to check the systems- it is better to check the afterburners on ground rather than finding any problems (like intermittent afterburner) in air. Of course in case of naval aircraft, the afterburners are on till the aircraft clears the ship.
Also, as @Jörg  pointed out, people upload videos that look 'cool'- hence the impression that they always use afterburners rather than the boring military power takeoff.