why GPUs, generally are in another electrical board (video card) and are not like CPUs to be installed in the motherboard?
for the consumer market, as in the average person building a home pc from parts acquired from newegg or amazon, yes it seems like GPU's are not like CPU's in terms of their installation into a computer - with the GPU being on a separate piece of hardware (the graphics card) that plugs into the motherboard via PCIe slot (and use to be an advanced graphics port (AGP) back in the day).
But a GPU is very much like a CPU, in terms of die form factor. And there are direct motherboard installations. The best example being the SXM2 form factor from Nvidia. You won't find this in the consumer market, this is in the enterprise / corporate realm where you are looking at a rack mount style server that costs upward of $50,000.
Dell has a model C4140 for example that incorporates some number of SXM2 style Nvidia gpu's, V100 model and later. I'm sure there are similar offerings from other big manufacturers like HP, Supermicro, and so on.
The gpu is however many mm in length and width, and dissipates N watts, and requires cooling, just like a cpu. Whether it's on a separate piece of hardware with it's own heat sink and fan, and is modular and plugs into the mainboard via pcie slot... or is incorporated onto the mainboard and has a heat sink on top of it just like the cpu does and uses case fans for cooling, is just form factor and is trivial.
we get gpu incorporated in to the cpu currently, with Intel's integrated graphics for example, so you can have a motherboard that has vga/dvi/hdmi/dp ports on it and you're not required to install a dedicated graphics card to get graphics. Will the consumer market ever see higher end gpu capability that sort of comes with on-motherboard gpu like the SXM form factor? with just 1 gpu there's no real good reason to go SXM style over modular pcie graphics card. SXM shines over pcie when you have multiple gpu's doing hard core gpu type of [cuda] processing, not just rendering graphics to a monitor to play a game. but who knows.