I've never had a problem building challenging encounters at any level. Yes, you need to think tactically and (as has been stated here and in other threads) you need to adjust the game to your players and the options you allow. The basic encounter guidelines seem assume no magic items, a non-optimized group, no feats. So you need to tweak them.
If you have specific issues with running the game at higher levels post them and we can give advice.
Some of the things I do
- If I don't want people to be able to teleport to safety, then teleportation is not allowed where they are (Hallowed or Forbiddance for example).
- If my monsters have a hard time hitting I either give them the effect of a Bless spell or have low-level mooks assisting. Goblins firing an arrow to do an assist and then hiding every round can make a big difference. If you care about XP budget (personally I ignore the number of opponents multiplier) you can throw in dozens of 1/4 CR monsters without upsetting the balance.
- Fighting a black dragon on it's home turf? Use Hallowed to make everyone vulnerable to acid damage.
- Getting Counterspelled constantly? Make your casters sorcerers with Subtle Spell so they can't be countered or have them casting spells from more than 60 feet away. Give them some way of hiding so they can duck into the shadows only to blast away from the darkness.
- Set up traps and environmental effects that aid the enemy.
- Have enemies come in waves.
- Be creative. Give the bad guys cannon fodder Bulette's (or purple worms, etc) that dig tunnels into the carefully defended area to launch a surprise attack from the rear.
- Don't always have the bad guys show up on a wide open plane in fireball formation.
- Don't give the PCs a break. Do the 6-8 encounters between long rests with only 1-2 short rests.
- Develop an XP budget that works to challenge your group. You may have to adjust the guidelines a bit (I keep standard for 1 group, multiply by 1.5 for another).
- One guy always hides in the back? Ambush him with enemies that flank the group now and then.
And so on and so forth. In addition, some challenges have nothing to do with battle but are ethical or strategic. A big enough army of orcs can still take out any group of PCs. Just don't have the orcs camp above ground in a known location where they can all be Meteor Stormed.
I do create custom monsters, but I do that at all levels. And, of course there's always house rules to make your life easier if that's your style.
Most of all, adjust the game to your players and have fun. Oh, and don't listen to the naysayers. Some people just live to whine.
Remember that the game is about having fun. The PCs should shine now and then, but also feel free to throw a horde of Balors at them if that's what you need to do to to challenge them.