Started writing this before I saw it had already been answered, but Imma post it anyway...
The short answer is, licensing issues.
Sony has held the rights to the character for years: they made the unconnected-to-the-MCU Tobey Maguire trilogy back in 2002–04, and then the relatively poor (IMHO, naturally) Andrew Garfield reboots in 2010–12-ish (basically to avoid having the licence revert to Marvel). But none of those 5 movies are 'necessary' to the MCU. AFAIK, Sony still
technically hold the rights, but were paid dollops of cash by Marvel(/Disney?) to allow the Tom Holland films: the "
Homecoming" subtitle is a deliberate reference to this.
Holland-as-Spidey shows up in
Civil War (which is
technically the third Cap movie, but essentially another Avengers movie, between
Age of Ultron and
Infinity War), so you might want to watch
Homecoming before
Civil War (even though it was actually released later, IIRC).
But
Far From Home happens after
InfinityWar/Endgame.