Scarecrows have a long history around the world, going back to ancient Egypt. Essentially, they are a technical solution to the problem of birds and mammalian pests in agricultural fields. Traditional scarecrows (humanoid decoys placed in fields, with or without noisemaking attachments) were widely used in the 1800s heyday of American family farming -- and although you can still see them on smaller family farms, improvements in 20th century technology offered new and more effective options.

Some of these technical advancements include:

Pesticides: Although not specifically designed to deter birds, pesticides such as DDT (1950s) and neonicotinoids (1980s) are harmful to birds and negatively impact the local bird population. (Needless to say, large scale application of pesticides presents a host of other environmental and ecological problems.)

Bird cannons: Similar to the devices used at airports to clear the air around runways, these devices emit a loud boom to drive away birds and crop pests.

Robots and drones: Fake birds of prey such as RoBird or the ProHawk drone, which deter birds and can emit the sounds of bird distress calls. The simpler, analog version of this idea is hawk kites -- which are maybe most closely related to traditional scarecrows.

Lasers: A relatively new technique (University of Rhode Island, 2000) that uses constantly moving beams of green laser light to scare off birds like starlings and red-winged blackbirds.

Ultrasonics: Often motion-activated, devices that emit ultrasonic waves to deter birds and mammalian pests. (For example, Ortho's Scarecrow product.)

In short, 19th century scarecrows just aren't up to the task of protecting contemporary agriculture at a commercial scale. They fell out of favor as better technical solutions became available.

(P.S. Links to a few specific products here because they best answer the question — I of course have no stake or interest in any of the companies or products mentioned.)