Because it is assumed that the consumption and investment relations in Equation (Y=...etc) are linear, ZZ is, in general, a curve rather than a line. (snap nog niet helemaal waarom maar...)
ZZ is drawn flatter than a 45-degree line because it is assumed that an increase in output leads to a less than one-for-one increase in demand.
''... When output increases, the sum of the increase in consumption and the increase in investment could exceed the initial increase in output.''