The thing is that simulation is not Xilinx expertise. Xilinx are FPGA developpers, and as such, concentrate on FPGA and tools required to map a design in their FPGA.
They do have a synthesizer, but there again, it may not be their main focus.
There are some companies that do specialize in synthesis and simulation. Those companies does that since many years and concentrate all their effort toward those products. They are recognized by the industry, and are often preffered by the customers designing the FPGA and ASIC.
For example, many ASIC companies require their custommers to at least test using some specialized simulators, like ModelSim.
So, for Xilinx interest, and for customers interest, Xilinx are better to put their efforts on their P&R and FPGA.
Well, I don't say they are not working on a simulator. They may well hire more resources to do that. But for the above reasons, costommers will take time to accept a new simulator. For most companies, for simulation, it is still "ModelSim and this", "That and ModelSim".