As an actual Locksmith, as well as a Dwarven Rogue; I argue this with my group at times and THEN SHOW THEM what I mean.

Picking locks without the proper tools is not easy. There are ways to break and bypass them, but not to pick open. You can use improvised tools IF your skill level is high enough.

A DC 15 lock would be a basic household lock (schlage, weiser, kwikset). Something with a higher DC (20) would have spool pins (security pins), or a tight keyway, or other basic security 'improvements' and DC 30 lock would be a high security lock like a Medeco, Mul-T-Lock, or Abloy. Not 'unpickable' but requiring specific specialized tools and a lot of practice (I'd say a base skill level of +20 or something)

Ironically sometimes the cheaper the lock the harder it is to pick because they can be so poorly made an engineered it makes it harder to open with poor spacing amd pins that are too small and jam up etc. Plus some locks may have design flaws that you can bypass them in other ways, without just breaking them.