Why should we need DAC ICs?
What you have right there is what is known in the field as an R-2R DAC, one of the many different kinds of often employed digital-to-analog converter topologies. You have answered your own question: why do we need DACs when we have this DAC topology? Because it is a DAC!

R-2R DACs purely by themselves are not great as a general purpose digital to analog converter. The output impedance of an R-2R DAC is very high, which means that the bandwidth will quickly be very limited. Even a few tens of picofarads capacitance on the output will reduce effective bandwidth and increase settling time to the MHz region. And this is equally true if you buffer the output with an opamp follower - well-trimmed opamps don't come in sub-pF input capacitances, and reducing the R-2R ladder resistances quickly increases power consumption to the point where it is unacceptably high. Don't get me wrong, there are super high bandwidth R-2R DACs on the market, but these are the kinds of chips that you find in arbitrary waveform generators in some scopes, and they have a bit heatsink and fan on top of them. 

There are other tradeoffs you can do with other DAC topologies. For instance, delta-sigma DACs do not to have a precision buffer output opamp and thus can be extended to very high bit depths (24-32 bit), where R-2R - because of the output buffering criterion - seldomly surpasses 12-bit. Successive approximation is another topology that is used, which inherently has a sample-and-hold on the output that can be driven with extremely low impedance (the same reason why conversely SAR ADCs can have a very high input impedance).