Lisa and Mac were products of his seeing the Smalltalk GUI at his visit to PARC. There was nothing off-the-shelf, so they had to be built from scratch.
Of NeXT he said that he had been so bamboozled by the GUI at his PARC visit that he missed the other two, arguable more important concepts: OO and networking.
NeXT used as much off-the-shelf components as possible: Ethernet + TCP/IP for the network, Unix for the OS, Adobe's Display Postscript for graphics, Stepstone's Objective-C for the OO parts (which in turn mashed together C and Smalltalk). It bundled TeX, Sybase SQL Server, a bunch of scripting languages, Webster's dictionary, etc.
They only built themselves what they absolutely had to to get the machine and user experience they wanted.