What made the developer's team to decide not to have a screensaver option on the elementary OS settings?
Not being one of the devs I can't really answer for them, but I suspect the reason is that screensavers are somewhat obsolete.
Unless you have a really old CRT display it is far better to allow the power saver to kick in instead. That saves the screen better than any screensaver and it saves energy.
There is nothing stopping you from apt installing the screensavers if you really want them though.
Here's Daniel Foré, the founder of elementary, on screensavers (see this bug):
I think the general consensus, last time it was discussed, was that it was better to just turn the display off.
As stated before, the periodic table has nothing to do with our brand.
I'm leaning towards this being a won't fix. Screensavers are kind of an outdated concept, IMO