Warning - long, nostalgia and rage fuelled wall of text ahead:
It wasn't always that way for Arty.
Once upon a time, SPGs were actually cared for, supported, and valued as members of the team. It was a magical time. People would go out of their way to scout for SPGs - but also protect them, knowing that they were useful - but - and this is very important - that SPGs could not win a game. They could help - but the skill/stupidity of a team's arty would not automatically lead to victory. SPGs would - usually - support teams with high damage output, rather than sniping a kill which was on 4%.(This last one infuriates me, especially if there are other targets visible)
Sadly, that time is long gone. The player-base became massive, SPGs were unintentionally buffed in a few patches, and more people started playing them. With more people playing as arty, you started to see idiots with increasing regularity, instead of the "old guard" of arty players, who actually knew what they were doing. Rather than 1-2 SPGs per team, we started seeing frankly insane 7-8 SPGs a side (now this has been limited to 5). Combined with their bigger and more powerful guns, SPGs became the scum of the earth. Even in games with a sane 2-3 SPGs on each team, higher tier SPGs could - and often did - win games. In fact they still do. In games of tier 8+, which have more than 2 SPGs each, it's usually the skill of the arty which decides the outcome.
Add in the fact that SPGs rarely go for sensible/strategically-wise targets, and it leads to more players thinking that arty are the scum of the earth, and deciding to let them rot (and laughing when the SPGs get hunted down). By this, I mean the "kill stealing". SPGs are about damage output, not kills. So it is incredibly infuriating to see the heavy tank you and 2 others have slowly been killing, get sniped at the last minute by some bastard on the other side of the map (who hadn't shot at him once) - when he really should have been shooting for something more important - like the enemy's SPGs, or TDs...
This ignores the inevitable friendly-fire. Usually occurs when the player tries to pull off a mind-numbingly stupid shot. Here's a tip (relevant not just to WoT) - you have 2 targets - one is danger close for an ally, the other is not. If there's a strong chance of you hitting (and killing -_- ) your buddy, it's probably best to go for the other target... Especially if the weapon you're using is a howitzer, which has a very large amount of splash-damage.
TL;DR - why don't most people look after or support their arty? 2 reasons - 1)morons, morons everywhere. 2)Due to point 1, people just stopped caring about arty, due to the likelihood that your SPG players were going to be as much use as a paper umbrella in a monsoon.