Better roads lead to improved transportation costs
Better ports lead to an ability to import and export more goods
More and better metro systems decreases traffic leading to better transportation times
Building industrial parks open up areas for large businesses
And obviously cutting taxes means people will be able to spend more and create jobs that way
How does having an improved infrastructure not lead to jobs being created? Even in the private sector
Look at the Transcontinental Railroad
Look at the interstate highway system
Look at the internet
And look at government investments in technologies that are improved upon by the private sector and later adopted by the public
Jet engines anyone?
How about all the things that have come from the NASA program?
Curiosity "10 NASA Inventions You Might Use Every Day"
Though NASA did not directly invent these things, they provided the demand and capital for these products to be improved upon and later released to the public for different uses.
Jobs and careers may be created by the private sector, but without investors those jobs would not exist. And government is capable of investing in programs they recognize as being good ideas.