SBR NBA odds history- why only even point spreads?

[monkeyspanker mo]

It's been a while since I checked the NBA odds history, but just noticed that all the current point spreads ended up on even (for example 4 or 5 as opposed to 4.5). Anyone know why this is

[Whippit]

or why 4.5 and not 4.25 I've often wondered