Your assumption is incorrect. Filters are sometimes placed at the back of the lens. Many lenses only support front filters, many only back ones and some even support both. Manufacturers prefer to use filters which go in the front because they are more convenient:

• Easy to add and remove.
• You can interact with the filter simply.
• You can stack multiple filters since there is room, up to a point where it causes vignetting or degradation.
• Minimize risk of dust entering where it would be difficult to clean.

Sometimes they give up because the design of the lens does not easily allow filters in the front:

• Extremely wide and fisheye lenses often cannot accommodate a filter without vignetting.
• A bulbous element may stick out from the lens barrel, making front filters needing a collar.
• A wide front elements makes a suitable enormous filter very large and expensive.

In such cases, manufacturers often have a slot to slip in a thin gel filter. This occurs in bright telephoto lenses such as the Sigma 500mm F/4.5 EX DG which takes a 46mm drop-in filter at the back. There is no standard size here either, although conceivably, it may be possible based on the size of the lens mount. Still, there are no adapters between rear filter sizes, so you need different sizes for now.

You can have both, even though it is unlikely to find a suitably large filter. The Pentax DA 560mm F/5.6 AW supports 112mm filters in the front and 40.5mm in the barrel.

There is even a way to interact with specially designed internal filters. The Pentax 645 DA 25mm F/4 comes with a polarizer which can be inserted in the middle and there is a control-dial to rotate it on the holder. You can see it clearly here.