Because the ECM needs to know that the EGR valve has actually opened when its commanded it to. When the EGR valve is closed the engine can only draw air via the air intake. This volume of air is measured by the Mass Air Flow sensor. When the EGR valve is open the engine can also draw from the exhaust. This causes a reduction in air flow across the MAF which is detected by the ECM, and which its programmed to take into account. If the EGR system is blanked off the ECM will not see this reduction in air flow. It will then know something is wrong, and will also reduce engine power output to ensure NOx emission levels are maintained.

You will not achieve anything by blanking or disabling the EGR. If you have a problem you'll have to dismantle the air intake completely and thoroughly clean it.