I believe the base game and the rules suggested for encounter design (plus the WotC-produced adventure path books) are built around the idea of a 4-player game using the Basic Rules. Usually 1 strong, armored guy, 1 agile guy attacking from the shadows, 1 armored healer doing okay melee damage, and 1 spellcaster throwing the occasional area of effect spells. That's what they talk about at the beginnings of the books as the typical party, and that's how I think everything was written and balanced for.
If you play a game like that... the "default" Basic Rules game is probably pretty challenging.
Anything you add on top of that though is going to make things easier. Every additional PC ups the ease. Every non-Basic Rules class ups the ease. Every additional healer ups the ease. Every special feature, spell or item taken from non-Basic Rules sources up the ease. Every allowance for taking unhindered short and long rests up the ease.
Since I'm willing to bet that 99% of the tables out there are NOT running 4-PC Basic Rules games... all the games will seem easier if you try and run them "Rules As Written". Which means the one obvious thing...
stop thinking that playing Rules As Written is anything worthwhile or something to inspire to!
Up the challenge! Add monsters to that adventure path you are running! Stop trying to create encounters using the encounter and CR rules! Throw crap at your players over and over and over again and let the chips fall where they may!
Now of course, most of the people who will react negatively to that will say things like "I can't restrict my player's choices because they see the entire PHB as valid and I'd be a jerk for doing so" or "I'm purposefully using an adventure path book so I wouldn't HAVE to do all the extra work to create or build up the challenges for my players" or "I have a living world and I'm not going to just ADD extra monsters who weren't there previously because that's
cheating and my players would no longer respect me!" Which, hey... I get. I understand.
But if that's the case... then just accept the game is going to be easier for your players. You choose to go past the default game 5E is set up and relatively balanced around, so that's how it is. Your choice on how to handle it.