Windows 7: why doesn't a ssd needs defragging?

System Manufacturer/Model Number HP P6715f
OS Windows 7 Home Premium 64 bit
Motherboard MS-7613 (Iona-GL8E)
Memory 5GB DDR3
Graphics Card Intel HD Graphics
Sound Card Realtek High Definition Audio
Monitor(s) Displays HP 2211X
Screen Resolution 1920x1080

Keyboard HP USB Keyboard
Mouse HP USB Optical Mouse
PSU 250W
Hard Drives 1.5 TB Hard Drive

System Manufacturer/Model Number Ignatz Special; 4 speed manual gearbox; factory air conditioning; one of one
OS Windows 7 SP1, Home Premium, 64-bit
CPU Intel Sandy Bridge i5-2500, not overclocked
Motherboard Gigabyte H67A-UD3H-B3, full ATX
Memory 4 GB Crucial DDR3-1333
Graphics Card none; graphics are integrated on CPU
Sound Card onboard: Realtek ALC892; external: USB Behringer UF0-202
Monitor(s) Displays NEC 90GX2-BK 19" LCD
Screen Resolution 800 x 640

Keyboard Leopold Tenkeyless with Cherry Blue switches, USB
Mouse Logitech or Microsoft optical wired; either USB or PS 2
PSU Seasonic SS-560KM, modular
Case Antec Solo II
Cooling CPU: Scythe Big Shuriken; Case: Scythe Slipstream 800 & 500
Hard Drives System: Intel 320 Series SSD, 80 GB; Data: Samsung Spinpoint 103SJ, 1 TB; Backup: WD Caviar Green WD15EADS-00P8B0, 1.5TB
Other Info Power consumption of this system, including monitor: 68 watts at idle; 144 watts at full load