but i have one quesion why do they use electrolytic capacitors??
Inexpensive, low impedance, good energy density, decent AC ripple capacity (in specific models), reliable when manufactured and chosen properly.
The reasons why electrolytic capacitors fail so often in cheap PSUs are mainly due to low-quality electrolyte (mainly affects cheap Chinese knock-off brands like Fuhjyyu) and using under-specified parts. Cheap PSUs often use only one or two knock-off capacitors rated for 0.7-1A RMS ripple on 20A rails where there may be around 10A RMS ripple. Good PSUs are more likely to have 2-3 major-brand (non-knock-off) capacitors rated for 2.5-4A each which makes them far less likely to fail than the Chinese stuff.
Polymer caps with better performance (5-6A RMS ripple) vs size characteristics are available but they are considerably more expensive.