ethernet - Why does no small Rj45 port exist?

While I bought my last Tablet I noticed that no Tablet (and most smaller notebooks /convertibles) has a Rj45 port. Which I finde quite dissatisfying since I like to use RJ45 in numerous places. I think a reason could be that Rj45 is simply too big for a Tablet - it is downright massive compared to micro USB /HDMI etc. But that leads me to my question: Why is there no attempt to build a smaller micro-Rj45 port which could be used in constrained spaces?

There's actually a few elements to this. Firstly invariably, ethernet is an additional, add-on chip, rather than something that's part of an SOC. Without a compelling reason for it manufacturers probably prefer to save a few dollars by leaving it out when not needed. In theory you could use an alternate port design, I seem to recall a zenbook model with a hinged full sized ethernet port, HDMI could do ethernet in theory, and there's a zenbook model that seems to use its displayport as a ethernet adaptor(For some reason asus experiments a lot with this sort of thing)

Practically in most situations where you're using a small, ultraportable device, you probably would be using wifi rather than ethernet. If you absolutely must have it, there's always the old stand-by of using a usb ethernet adaptor.

Its really an economic thing, more than anything else. There's not much demand for a non poweruser for ethernet in devices too small or slim for a full sized port, and a non standard port would not be likely to catch on

I think there is a difference between Tablets and other types of computers like Desktops, Laptops and Servers.

Tablet's main aim to acquire mobility. So, Wifi or 3G is the most suitable connectivity method for tablet devices. Tablet manufacturers may not interested in giving a solution for your problem because that is not average tablet user's demand.

The above answers from Journeyman and U-map are correct that a general lack of demand combined with the size of the connector and, to a lesser extent, the cost of the Ethernet PHY chip are among the primary reasons for leaving these off of most tablets and thin laptops. However, especially where tablets and other devices with small batteries are concerned, power is also an issue. From my experiments on development kits, I've found that shutting down the Gigabit Ethernet PHY saves about a Watt. While that's not a big deal for most laptops, it's a much bigger deal with tablets (and especially phones.) Also, the fact that USB-attached Ethernet NICs are cheap and readily available for the relatively small subset of users that actually want to use wired Ethernet on their mobile devices makes the manufacturers be even less worried about leaving it off.

Smaller ethernet ports actually do exist. The XJack was a connector used on PCMCIA modems and network cards that folded up and retracted inside of the card when not in use. Instead of using a housing that completely surrounded the RJ45 connector, the cable connected at an angle and the jack had just enough material to make an electrical connection and hold the cable in place. It looked odd and was awkward to use at first, but it worked, and it didn't take up much space.

Type II PC cards are only 5mm thick, so the XJack would have been no thicker than that. That's plenty thin, even for today's thin devices. I think the reason that tablets/phones lack ethernet ports is not a matter of the connector. As other people have mentioned, it's the internal circuitry that device makers want to avoid. The extra cost (and probably more important, power consumption) associated with adding ethernet isn't worth the benefit. Using wired ethernet with a tablet is also contrary to their being marketed as "mobile devices", since you end up tethered to the wall.