Everyone does not make 1LT or CPT, but the selection rates are very high. An officer not promoted to Major must resign, Majors and above can stay to retirement.
When I was in, LTJG was automatic since no in flight training had yet been winged. Selection to LT was 85%; LCDR 65%, to CDR 55% and so on. Selection to CAPT was much lower; in my year group, it was 6%. Of course, the year you came up for promotion was also important.