In a military organization, there is more to leadership than ambition, charisma and so on. The leader must have a sound understanding of Tactics, Battlefield Operational Systems, Logistics, his units equipment, psychology, administration and many other things as well. Anyone can lead people into a debacle, few can lead to success. Military organizations develop and train thier leaders and don't rely on natural selection, it's far too costly -not only in economic terms but in terms of victory. The US Army has a stepped learship training program called NCOES: Primary Leadership Development Course at SPeCialist to SGT level, Basic Noncommisioned Officer Course at the SGT to Staff Sergeant level, Advanced Non-Commisioned Officer Course at the Staff Sergeant to Sergeant First Class Level, First Sergeants Academy at Master Sergeant to First Sergeant level and the Sergeants Major Academy for Sergent Major Level. There is a similar program for officers even though they all begin with at least a bachelor's degree in military science. Most advanced nations have similar programs, though america's is acknowledged as the best in the world and many send thier leaders to our training facilities whenever possible. The natural selection part is that unless you show apptitude for leadership you do not get selected for promotion and attendance to even the beginning rung of the training ladder. Also on parallel with the leadership training are stepped Occupational skill technical training in four (five) levels. level 10,20,30, 40 and sometime 50. Level ten is the level required for basic proficiency in a skill field (for example; how to set up, operate, and march-order an Artillery locating RADAR system) level 20 is for SGTs or team leaders, level thirty is for section chiefs/ squad leaders and level forty is for SFCs-platoon SGTS. Level fifty is for company,battery, battalion or above leaders. Each one requires additional proficiency in the levels below as well as new proficiencies in more complicated tasks. at one level you can operate at the next level, operate, troobleshoot, and so on. Eventually you are the person advising a commander as to what your type of equipment and troops can do, where they should be doing it, how and so on based on complex technical and tactical considerations such as what his mission and concept of operations is.
Even with the wealth of an entire galaxy, one couldn't field the type of navy shown in star wars. One star destroyer has thousands of technical and tactical positions requiring expert level proficiency. The fighter jocks don't just jump into a cockpit and rev 'er up. The ship itself costs billions of credits/dollars/ whatever. It takes months or years to build the ship. It takes time to train the crew. It takes time to run a shakedown of the ship. The ancilliary equipment and supplies including consumables all cost huge amounts. Trooper armor, arms, commo gear medical gear and other field gear. Support ships and fighters and so on. Logistical support personnel and facilities not even aboard the ship also cost big and are effected by the fate of the ship. Finally, stategic planning requiring the health of the ship are destroyed along with the ship should it become a causualty as other ships must be diverted if the task was critical.