Actually, there are quite a few Arty kits out there, you just have to look for them. Here are some US Arty pieces, I don't build or follow all the Ger WWII stuff, can't help there.
WWII Era, you have the great kits by AFV Club of M59 Long Tom 155mm and M115 8" howitzers. Academy does the M12 SP Howitzer. Italeri does the M101 105mm and M1A2 (M114) 155mm howitzers, and the M7 Priest 105mm SP howitzer too.
Vietnam era: Italeri M101 and M114 again. Italeri M109 155 SP (can also be built as an M108 105 SP), M107 175mm and M110 8". AFV Club M102 105mm howitzer.
The modern arena is especially well represented.
For Operation Desert Storm, a.k.a Gulf War, 1990-91.
US Army used M109A5, M109A6 Paladin was not fielded till after the war, about 1994. Italeri did make a kit of the A5 as well, you could actually build any version of M109 with the kit. It was listed as an M109A2/A4 kit, same vehicle on exterior. I think it is now discontinued with the intro of Paladin kit.
M109A5

M109A6 Paladin

Also used were the afore mentioned M102A1 105mm howitzer, available from AFV Club.

And the M198 155 towed Howitzer that is available from Mouse House Designs from Australia for a small fortune.
http://www.mheaust.com.au/MHE/Resin/OZA/M198.htmM198

Lastly, the big guns used are the M270 MLRS available from DML.
M270

USMC used both the M198 and also the M110A2 8" howitzer (203mm.), available from Italeri as well.
USMC M110A2

Some USMC units still used the old WWII era 105mm M101A1 howitzer as well, avialable from Italeri, may be out of production though.
M101A1

For Operation Iraqi Freedom, Iraq 2003-present.
US Army is using M109A6 Paladin, M119A1 105mm howitzer, M198 155mm howitzer, and M270 MLRS. Some Army National Guard and Reserve units are still deploying with M102A1s as well
M119

USMC is still using M198 exclusively. They have retired all other artillery pieces and rely on the US Army for additional fire support.
There are other countries' arty pretty well represented as well. Tamiya does a Brit 25 Pound howitzer. Eastern express does the D-30 122mm, and a 100mm gun. Skif does a BM-21, and a 2S1 122 SP. Italeri does the post WWII BM-13. DML has the SCUD-B. Trumpeter does the Brit AS-90 and a bunch of Chinese pieces, one that could be converted into a Soviet 2S3 152 mm SP pretty easily. Revell does the Panzer Howitzer 2000 155mm SP as well.
I'm sure there are more out there that I am missing as well. Hope that brightens up some of your outlooks on the state of Artillery models available.
Happy Arty modelling to all.