Voltage divider not functioning correctly
The MCP3002 datasheet  says:


  Ideally, the impedance of the signal source should be near zero.  This is achievable with an operational amplifier such as the MCP601 which has a closed loop output impedance of tens of ohms. The adverse affects of higher source impedances are shown in Figure 4-2. 


The maximum source impedance plotted in that graph is 10KOhm. You are using way above that (megohms). The datasheet quote above also says how you can fix that.