First of all, not all farmers have computers. Within the diffision of innovation process, most farmers tend to be late adopters or laggards. This rings true for farmers' rate of adoption for computers. Also, the farmer may not have the funds necessary to buy a computer; he may need to reinvest his profits directly back into his crop(s).
Secondly, the information a farmer may need might not be on the Web. Even though there is a ton of information on the Web, not everything you need to know about farming every crop is online.
And last, unless the farmer owns a laptop computer (not typical for most farmers), they will have to return to their office in order to obtain the information they need. This trip from the field to the office takes up time and money, neither of which a farmer can spare.