I honestly think I'd have zero interest in RPGs if they didn't have that inbuilt customizability. To me, someone who only uses official material feels so constrained as to defeat the point of the game for me. I know folks like that exist, but it's a mindset that is utterly beyond me.
I agree with this 100%.
Back when I first started we made up our own stuff, and that was before we discovered Dragon Magazine. I used to spend a good chunk of my money as a young teen photocopying pages out to collect everything together in one spot.
Later I felt using rules not in the core book was almost mandatory. I had players that owned and nearly memorized the Monster Manual (I & II), and the Fiend Folio, not to mention the magic items out of the DMG. To keep them on their toes, and keep a little mystery in the game I would homebrew or use stuff from other sources. Back then I would even bolt on pieces from other game systems. We used to use the critical hit tables from Middle Earth RPG. Heck, my 5e game uses 13th Age's Escalation Die.
Now, I'm an En5ider patron. I buy stuff on the DMs Guild. I have copies of Fifth Edition Foes and Tome of Beasts. I use homebrew material posted here. I read and use stuff from many different blogs. I make my own stuff, and convert stuff on my blog from those same old Dragon Magazines I photocopied so many years ago.
I have my Google Drive account set up like my own personal SRD, which I've shared with my players. I copy and paste blog entries into it. When I have time, I take apart PDFs to separate them into their own entries so I don't have to search through books for them. When I run games, I commonly substitute monsters. Enemy spellcasters use new spells, and rarely have my players found a magic item from the DMG.
Usually I give stuff a read thru, and if I like it, I use it. Player options are available with the understanding that they are "playtest material" and that we may tweak, change, or completely discard stuff at any time. Same with any magic items and spells I hand out. I've changed monsters because they were not working as intended as well.